git decorations

[danprince]

Closes #2. Not entirely happy with this yet. Can't find a clean way to line up to exactly what the built-in file explorer uses to check Git status. Also slows down the extension's boot time, maybe to do with loading the git stuff?

Probably needs a read of the actual File Explorer implementation before it can be wrapped.

[danprince]

The underlying file explorer mostly seems to collect its decorations through APIs that I can't access.

Here's the FileDecorationProvider from the Git extension.

As far as I can tell, that provider isn't exposed through the extension and neither is the underlying repository class that exports those changed resources. Only a subset are re-exposed publicly.

I'd have to rely on private members to get the rest and it just doesn't feel worth it, considering I have no plans to make any other git operations work.

Will just chalk this one up to "nice to have" and revisit in the future if any of those APIs open up.