Use the raw external definition to check parsing as test parameters
identifier to get a more meaningful pytest output.

When an external is defined using legacy format (svn < 1.5), the official
subversion client automatically uses the peg_rev parameter of the export
operation so ensure to have the same behavior in the loader to avoid hash
mismatches in reconstructed file systems.

It simplifies code, improves readability and facilitates the adding
of new data related to a parsed external definition.

When a path is copied using a copyfrom operation, externals set on the paths
being copied must also be set on the copied paths.

Previous implementation was using the latest externals values set on the
paths being copied but those could differ from the ones set at revision
copyfrom_rev so ensure to set correct externals on the copied paths.

Renaming the celery task broke the subversion loading in production as the
load-svn task type is already registered in scheduler database and targets
the previous celery task name.

Related to swh-environment#3925.

Convert README from markdown to ReST to make it embeddable in
docs/index.rst

It was broken in c834321d likely
due to a side effect related to a local configuration.

This is what is actually done on production system (via custom
configured scheduler task type definitions).

This also adds a commit to fix the svn export related task and loader inconsistently
named.

Refs. swh/infra/sysadm-environment#4906

Otherwise, we'd lose the context in the snapshot.

Refs. swh/meta#4979

Refs. swh/meta#4979

Refs. swh/meta#4979

Some remote subversion repositories in the wild require to use anonymous
credentials to obtain a read access. Those credentials can be either
'anonymous/' or 'anonymous/anonymous' in most of the cases.

Previously, it was required to add the credentials in the origin URL using
basic authentication syntax. While such URL can be provided to the Save Code
Now service, it cannot when svn URLs are coming from a lister.

To workaround this, try to connect using these anonymous credentials in the
get_svn_repo function when a connection error happens.

This should also simplify the submission of Save Code Now requests when
submitting a subversion origin that requires anonymous credentials.

Previously when exporting a sub-path of a remote subversion repository
over the network, the full repository was exported and the local path
targeting the sub-path was returned. This is no really optimal in terms
of network bandwidth if the repository filesystem is large but it was
implemented like this to ensure all tests related to sub-paths export
were passing regardless the subversion loader class used: either SvnLoader
or SvnLoaderFromRemoteDump.

After some analysis, it turned out that it was possible to avoid exporting
the full repository but only the request sub-path when using the SvnLoader
class. So modify the SvnRepo class to ensure that behavior and save some
network bandwdith when dealing with a large repository.

These changes in the SvnRepo class induce some in the replay module to ensure
all tests still pass and it also enables to remove a no longer needed optional
parameter to the class constructor.

This overrides the default core class DirectoryLoader's fetch_data method to export an
svn tree at a specific commit or tag. The remaining behavior is the same as the
BaseDirectoryLoader. Checks the checksums of the tree is ok, then ingest the DAG objects
into the archives (including the NAR as extid).

Refs. swh/meta#4979

Add jenkins badge for master branch build status.

Rephrase introduction sentence.

Remove remainings from when the file was written in restructuredText.

Add syntax highlighting to code blocks.

The export_temporary method of the SvnRepo class exports the content of
a subversion repository at a given revision in a temporary directory.

As we also export the externals that might be associated to some paths
in the repository, we first need to get all the svn:externals property
values in order to determine if there is recursive or relative externals
and adjust some export parameters accordingly.

While that operation is fast when the subversion repository is hosted
locally, it is terribly slow when the repository is hosted on a remote
server. Indeed a recursive propget operation on a remote server sends
a lot of network requests which slows down quite a lot the process,
especially with large repositories.

To improve the performances, the previous implementation was doing a
full checkout of the repository to local filesystem and gets svn:externals
property values from it. Nevertheless, that process is time consuming for
large repositories and it can consume a lot of disk space.

In order to remove that bottleneck and improve overall performances for
getting all properties values, introduce a C++ extension module for Python
that implements a fast way to crawl all paths of a repository and their
associated properties. Unlike "svn ls --depth infinity" or "svn propget -R"
commands it performs only one SVN request over the network, hence saving
time especially with large repositories.
The code is freely inspired from the fast-svn-crawler project by Dmitry
Pavlenko (https://sourceforge.net/projects/fastsvncrawler/).

The obtained speedup is quite impressive, on a large remote repository
listing all paths using "svn ls --depth infinity" or gettings all svn:externals
property values using "svn propget -R" takes around one hour while it takes
only a couple of minutes using the approach implemented in the C++ extension
module. Using that approach also enables to save disk space as we no longer
need to perform a full checkout of the repository.

This change should greatly improve the performances when reloading a svn
repository already visited by Software Heritage. Indeed, before the possible
archiving of new commits issued since last visit, the loader checks that a
repository has not been altered by calling the export_temporary method using
the remote repository URL.

Some external definitions can have leading or trailing spaces/tabs so we
need to strip them to avoid parsing errors.

Fixes #4734

Official subversion documentation only mentions that paths containing
spaces must be surrounded by double quotes but we can find some external
definitions in the wild whose paths are surrounded by single quotes.
Those are properly handled by the official subversion client so we must
do the same when parsing externals.

When a directory is copied from another one in a previous revision,
externals must be copied only if they have been defined in a revision
greater or equal to the revision the directory is copied from.

So store the revision number an external is defined and use it to filter
externals when performing copyfrom operations.

As the swh.loader.svn.svn_repo.SvnRepo class quote all URLs before calling
subversion API through subvertpy, me must ensure to unquote URLs extracted
from external definitions otherwise they will be double quoted and thus no
longer valid.

When a directory is deleted by subversion, we must also remove its
state holding externals info as the directory can be re-added later
in another revision but without the svn:externals property set.

Previously SvnLoaderFromRemoteDump class was using the repository
root URL to dump a sub-project.

"svnadmin load" has a --no-flush-to-disk option enabling faster load
while being unsafe on power off. This drawback is not an issue for the
subversion loader so use that option to significantly improve the
performance for loading a repository from a dump file into a directory
on the local filesystem.

Those methods ensure URLs are properly quoted to avoid assertion failures
when calling functions from the subversion C API.

In order to ensure consistency between SvnLoader and SvnLoaderFromRemoteDump
classes, run most of the tests with both of them.

As a consequence, fix an invalid load status that was reported by the SvnLoader
class when no new objects to archive have been found during a visit.

That kind of error can be encoutered when loading a repository hosted on
SouceForge.

Recursive propget operation is terribly slow over the network,
better doing it from a freshly checked out working copy as it is faster.

Related to swh/meta#4960

Related to swh/meta#4959