hg-to-git: --allow-empty-message in git commit

Do not fail to import mercurial commits with empty commit messages.

Signed-off-by: Maurício C Antunes <mauricio.antunes@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/hg-to-git/hg-to-git.py

@@ -225,7 +225,7 @@ for cset in range(int(tip) + 1):
     os.system('git ls-files -x .hg --deleted | git update-index --remove --stdin')
 
     # commit
-    os.system(getgitenv(user, date) + 'git commit --allow-empty -a -F %s' % filecomment)
+    os.system(getgitenv(user, date) + 'git commit --allow-empty --allow-empty-message -a -F %s' % filecomment)
     os.unlink(filecomment)
 
     # tag