subrepo: use low-level git-diff-index for dirty() Despite its name, git-diff-index compares a revision to the files in the working directory. This seems way less sketchy and more future proof than parsing human-readable git-status.

subrepo: defer determination of git's current branch

subrepo: incorporate tracking branches into gitbranchmap

subrepo: use low-level git-for-each-ref command in branchmap This command's output doesn't change across versions, and it also disambiguates cases where there are slashes in local branch names.