Add git awareness to file tab icons

[nesrve]

Before:

After:

Release Notes:

• N/A

[SomeoneToIgnore]

(also, the Before image seems to be wrong)

[SomeoneToIgnore]

Ok, after the recent release, we have a week to get the feedback based on nightly builds and consider leaving or reverting this.

[ghost]

LGTM code-wise, let's wait for the design notes.

As a potential follow-up, we might consider the same approach for file_finder items. There are no icons and no git statuses there yet, maybe it's a good time to consider adding those too if @iamnbutler has an opinion?

i could work on this too and open a separate pr, if it's something that's wanted

[SomeoneToIgnore]

I cannot be sure as no feedback on the design was given, but personally interested in looking at how it looks, so can review the new PR.

I could swear there was an issue for this, but failed to find it now.