automation: add check that hg source directory is a repo
Synchronizing from e.g. source distributions is not yet supported.
Let's add a check so we fail with an error message indicating
such.
Differential Revision: https://phab.mercurial-scm.org/D6315
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state