hg: make `clean` return consistent with the `update` function

The function return a boolean and is used as such. Lets be explicit about it.

Differential Revision: https://phab.mercurial-scm.org/D10156

mercurial/hg.py

@@ -1121,6 +1121,7 b' def clean(repo, node, show_stats=True, q'
     assert stats.unresolvedcount == 0
     if show_stats:
         _showstats(repo, stats, quietempty)
+    return False
 
 
 # naming conflict in updatetotally()