Show the sha1 of the updated files in the debug logs
Fix the git backend when the remote repository goes back in time

That's the first bug discovered by `opam-rt`. Yeah!
Fix the rsync command

* copy symlinks as links (with `-L` instead of `-l`, thx to @trefix for the fix)
* use the file checksums instead of relying on mod-time and name (thx to `opam-rt`)
* do not sync the .hg subdir (if any)