attempt to avoid content differences due to paths in keywords

Some RCS keywords, such has "Header", contain absolute file paths derived from the on-disk filesystem path of the CVS repository.

When we fetch files over the pserver protocol such keywords are expanded by the CVS server. But when using the rsync protocol we will first copy the CVS repository to local disk and the path to this local copy will correspond to some temporary directory.

Try to avoid file content differences between pserver and rsync access methods by deriving a likely server-side path from path information found in the rsync:// origin URL. This will work as expected as long as the CVS server-side setup exposes the same path to the CVS repository over both access methods, which is the case for GNU savannah for example.

In general, we should recommend treating pserver and rsync as distinct origins and not rely on them to be interchangable and always produce the same conversion result. But we can still try our best to avoid needless differences in content hashes.

Migrated from D6678 (view on Phabricator)