convert: handle LookupError in mercurial_source.lookuprev()

This is in line with the documentation on the base class method, and is related
to issue4496 (but doesn't fix the reporter's problem of not mangling other data
that matches a revision pattern).  Now instead of aborting when there is an
ambiguous source rev, it simply won't update the commit comment.  A warning
message might be nice, but a None return masks whether the problem was no
matching revision, or more than one.

The only other caller of this is the logic that converts tags, but those are
never ambiguous since they are always 40 characters.

A test isn't feasible because there simply aren't enough commits in the test
suite repos to have an ambiguous identifier that is at least 6 characters long,
and it would be too easy for the ambiguity to disappear when unrelated changes
are made.  Instead, I simply ran 'hg --traceback log -r c' on the hg repo, and
handled the error it threw.

hgext/convert/hg.py

@@ -467,7 +467,7 b' class mercurial_source(converter_source)'
     def lookuprev(self, rev):
         try:
             return hex(self.repo.lookup(rev))
-        except error.RepoError:
+        except (error.RepoError, error.LookupError):
             return None
 
     def getbookmarks(self):