This will decrease the disk pressure currently existing when ingesting a svn repository
out of a remote dump. We first fetch a dump, then mount a svn repository out of it, and
at last we ingest the repository and have a growing copy on disk . So we are up to 3
copies which takes lots of disk space.
With the following commit, we take down the 1 unnecessary copy (svn dump) as soon as
possible (when the svn repository is mounted which we ingest).
 implementation detail for speed, we iterate over the commit log, apply deltas (and
computing swh hashes) along the way on disk.
- improves the existing docstrings
- add types
- add tests on missing utils part (currently tested indirectly)
Related to T3719