We can use `format_git_object_from_parts` inside it.

This would be useful for the IPFS bridge, and seems good to complete the
API in any sense.

That hook can be frustrating as it can discard a long commit message
if it finds a typo in it so better removing it.

it's a piece of information used several times in the swh stack.

Related to T3922

black is considered stable since release 22.1.0 and the version
we are currently using is quite outdated and not compatible with
click 8.1.0, so it is time to bump it to its latest stable release.

Please note that E501 pycodestyle warning related to line length
is replaced by B950 one from flake8-bugbear as recommended by black.
https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#line-length

Related to T3922

Hypothesis: This makes the documentation build fail.

This changes the deprecated :show-nested: instruction to the :nested: one [1].
This also fixes warning about misdefined block [2]

[1] https://sphinx-click.readthedocs.io/en/latest/usage/#directive-click

[2]
```
10:38:38  Warning, treated as error:
10:38:38  /var/lib/jenkins/workspace/DMOD/tests-on-diff/docs/cli.rst:14:Literal block expected; none found.
10:38:38  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
10:38:38  make: Leaving directory '/var/lib/jenkins/workspace/DMOD/tests-on-diff/docs'
10:38:38  ERROR: InvocationError for command '/usr/bin/make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs' (exited with code 2)
```

committer=None happens on some malformed commits generated by old dgit
versions ; and it is possible for author=None to happen for the same reason.

Due to test modules being copied in subdirectories of the
build directory by setuptools, it makes pytest fail by raising
ImportPathMismatchError exceptions when invoked from root
directory of the module.

So ignore the build folder to discover tests.

these fields are computed attributes and may be removed from the backend
storage. Helps writing tests.

This will be used by swh.storage.backfill's tests.

This reverts commit a0f54362.

This means this commit removes the 'offset' and 'negative_utc'
arguments.

It also removes the 'negative_utc' attribute (which is not used
anymore), but keeps an 'offset' property, which is an alias to
'offset_minutes()'. This is only to keep this commit readable;
the next commit will remove this alias.

To install the new hook:

  $ pre-commit install -t commit-msg

This should slightly reduce the memory used by dir/rev/rel objects

A future commit will remove the 'offset' attribute, and rename
'offset_bytes' to 'offset'.

Modules using the `.offset` attribute will need to switch to this new
method to keep working.

Sphinx >=4.4.0 raises warnings (which are errors for us) when using
warnings instead of an extlink role defined in the configuration

This allows keeping compatibility with existing users of the TimestampWithTimezone constructor

'offset' becomes a property instead of an attribute and constructor argument.

This also removes both from the output of `.to_dict()`.

This is step 6 of https://forge.softwareheritage.org/T3752

This will break packages that still use the constructor directly,
ie. swh-storage and swh-loader-git (and tests of swh-loader-core
and swh-loader-svn)

swh-deposit needs to link to some of them.

This is needed when recomputing the manifest of git objects with weird offsets.

blake2s and blake2b have been provided by the stdlib hashlib since
Python 3.6, and we declare 3.7 as minimum Python version supported.

1. Most objects do not need it so it's a waste of space
2. This means we just extend the existing format (some objects will
   have that key in their dict) instead of changing it (retroactively
   adding it to all objects)

This will be used to store the original manifest of 'weird' git objects,
when we cannot reasonably represent them otherwise.

We don't store those

Revision and release do not generally allow 'arbitrary' metadata;
and it was missing ExtIDs and REMD