This will help in debugging issues as rustfmt gets more users.
If the working tree is clean, output looks like
$ target/debug/rustfmt -V
0.5.0 (9f5ed3b)
If the working tree is dirty, output looks like
$ target/debug/rustfmt -V
0.5.0 (9f5ed3b worktree dirty)
If git is unavailable, output looks like
$ target/debug/rustfmt -V
0.5.0 (git commit unavailable)
To avoid rebuilds on changing tests, the build script will only rerun if
files under src/ are changed. This means the actual git status may show
changed files and this would not show up in the version. This should not
be an issue as files not in src/ should not affect the build output.
