git: Ensure no backend ever takes an index lock

[bgw]

git status (and git describe --dirty) takes an optional write lock as a performance optimization: https://git-scm.com/docs/git-status#_background_refresh

This is bad because:

• Other processes might run at the same time, try to acquire the lock, and fail.
• The child process can get killed for a variety of reasons, and may not be able to clean up the lock, leading to later errors about stale lockfiles.

Both of these situations are very hard to root cause and debug.

git describe does not currently support GIT_OPTIONAL_LOCKS=0 (gitgitgadget/git#1872), so this reimplements the -dirty suffix logic using git status.

This is not an issue for libgit2 or gix, but this includes tests for those backends in case they change their implementations.