git: Skip directories in git status output (#23300)

The output of `git status --porcelain=v1` includes untracked
directories, i.e. directories that have no tracked files beneath. Since
we have our own way of computing a "summary" status for each directory
in a repo, this is not helpful for Zed; and it interferes with our
handling of nested repos. So just skip these lines in the output.

Closes #23133 

Release Notes:

- Fix project panel colors when one git repository is nested beneath
another