Implement Git as another VCS backend

[ermshiperete]

This depends on #192.

[rmunn]

One issue is that we use branch names to track FLEx model versions, e.g. 7500002.7000072. In Mercurial, each commit is associated with a branch, so "which branch is this commit from" is part of the Mercurial data model and is easy to retrieve. In Git, commits do not know which branch they're on, and it may be more difficult to work out which commit belongs to which branch: we'd have to start with the various branch heads and work backwards through their parent chains to assign branch IDs to revisions.