clean: check that there are no conflicts after

As noted by Pulkit, there should never be any conflicts after doing a
clean update, so `hg.clean()` should never return `True`. Let's check
that assertion instead to clarify the code. The callers will now get a
`None` instead of a `False` returned, but that should be fine (both
result in a 0 exit status).

Differential Revision: https://phab.mercurial-scm.org/D7984

mercurial/hg.py

@@ -1040,10 +1040,10 b' def update(repo, node, quietempty=False,'
 def clean(repo, node, show_stats=True, quietempty=False):
     """forcibly switch the working directory to node, clobbering changes"""
     stats = updaterepo(repo, node, True)
+    assert stats.unresolvedcount == 0
     repo.vfs.unlinkpath(b'graftstate', ignoremissing=True)
     if show_stats:
         _showstats(repo, stats, quietempty)
-    return stats.unresolvedcount > 0
 
 
 # naming conflict in updatetotally()