Improvements to _GitSourceBase and _GitMirror
This branch does some refactoring and minor feature additions to the Git source base class.
- The fetch is done with depth=1 when an available tag is given
- git-describe tag prefixes are stripped from the ref when calculating the cache key
- The mirror is not created with an initial clone, but is set up manually from an empty repo
- To simplifiy code and Git calls, remotes are no longer used; instead URLs are passed on the command-line
-
latest_commit_with_tags
is refactored into smaller functions