use group option into revert_sources
Improved readability by wrapping systematic logging
[git] refactored subsystem to allow for new features

* fetches are now just about the wished ref (branch or tag)
* official deprecation of direct SHA usage
* tests for tag use-cases
* split get_update() in smaller methods
* --depth support should be easy to add.
[git] introduction of --depth, testing some scenarios
[git] more testing related to --depth, and subsequent fixes
[git-depth] global option, and value validation
[git-depth] doc
leftover pdb
[git] fixing the offline mode after breaking it
[buildbot] added a bootstrap v1 for the buildouts that need it
[buildbot] no, bootstrap-v1 can't work (dead URL)
Setting versions for release
Merged the 'sober-git' branch for launchpad #1332465 and more optims

Also took care of broken test_clone because of nested working directory keepers
Merged 1.8.6 release info from stable branch
launchpad #1391571: reproduction by unit test (and found another minor bug
Merged fix for #1391571 by Laetitia Gangloff
Minor flake8 error (blank lines)
[git] more tests for branches and depth
launchpad #1394842: [git] fixing at least for 1.7.10
launchpad #1394842: [git] forgot that shallow clones also need remote tracking