Ignore warnings when running hg commands.

hg may sometimes emit warnings that are not actually meaningful to the
command being run by post-review.  For instance, my hg will emit the
following for 'hg -q outgoing --template 'b:{branches}\nr:{rev}\n\n'
when run with python2.5:

---
warning: certificate for foo can't be verified (Python too old)"
b:
r:8869
---

This warning is not meaningful, so we just ignore it.

Reviewed at http://reviews.reviewboard.org/r/2995/

rbtools/clients/mercurial.py

@@ -245,7 +245,10 @@ def _get_outgoing_changesets(self, current_branch, remote):
             if not pair.strip():
                 continue
 
-            branch, rev = pair.strip().split('\n')
+            # Ignore warning messages that hg might put in, such as
+            # "warning: certificate for foo can't be verified (Python too old)"
+            branch, rev = [l for l in pair.strip().split('\n')
+                           if not l.startswith('warning: ')]
 
             branch_name = branch[len('b:'):].strip()
             branch_name = branch_name or 'default'