Add git workflow

[sirbrillig]

This adds a --git CLI shortcut and a git workflow to automatically collect the three necessary pieces of data for a git-versioned file in the same way that the --svn shortcut does for svn-versioned files.

• Add tests
• Add workflow
• Add CLI command
• Add docs
• Handle untracked and unchanged files

Fixes #3

[sirbrillig]

This needs to more gracefully handle untracked (by git) files and files with no changes. Those should both have test cases.

[sirbrillig]

There's two skipped tests that are just not working for some reason. I'd like to get them to run. It's something to do with exception handling. I think the actual CLI works correctly.

[sirbrillig]

🤔 now untracked git files are not being reported at all without --debug. That's no good.

[sirbrillig]

Now I'd like to add svn workflow tests to match the git ones.

[sirbrillig]

Ok, I think this is in good shape. I'd like to clean up the commit history and do a little more real-world testing before merging.