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

contrib/automation/hgautomation/windows.py

@@ -156,6 +156,10 b' def synchronize_hg(hg_repo: pathlib.Path'
             fh.write('  UserKnownHostsFile %s\n' % (ssh_dir / 'known_hosts'))
             fh.write('  IdentityFile %s\n' % (ssh_dir / 'id_rsa'))
 
+        if not (hg_repo / '.hg').is_dir():
+            raise Exception('%s is not a Mercurial repository; '
+                            'synchronization not yet supported' % hg_repo)
+
         env = dict(os.environ)
         env['HGPLAIN'] = '1'
         env['HGENCODING'] = 'utf-8'