lib.commitIdFromGitRepo now resolves the refs from the
parent repository in case the supplied path is a file
containing the path to said repository. this adds support
for git-worktree and things alike. see gitrepository-layout(5).
this also:
- adds a new boolean function lib.pathIsRegularFile to
check whether a path is a regular file
- patches lib.revisionWithDefault and
the revision and versionSuffix attributes in
config.system.nixos in order to support git-worktrees
c9214c394b