re_analytics: track git hash's dirtiness in real-time

[teh-cmc]

Right now we're watching .git/refs/heads/** to detect hash changes, but of course this is not enough to detect changes to the repo dirty state as they happen.

@jleibs came up with an evil plan for that:

You could do it with a two-stage process
one build.rs that conditionally updates a file if it changes
and a second build.rs that watches for changes on that file

[emilk]

Related: #1388

[teh-cmc]

We don't keep track of dirty state no more.