lib.commitIdFromGitRepo: support git-worktree

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