UpmGitUrlCommand: Handle local git repos (file protocol) and tags

[chrisyarbrough]

https://docs.unity3d.com/6000.3/Documentation/Manual/upm-git.html#syntax

Unity supports adding git repos from files. Not a common use-case, but potentially something to let the command handle.

Ideally, we would check:

• tag (but what if there are multiple?)
• branch
• revision

And handle the case where the git repo is just initialized and doesn't have any commits yet.
In theory, a git repo could also have no branches, but I'm not sure if Unity would even work with that.