hg: add quietempty flag to _showstats

hg: add quietempty flag to _showstats

if called with quietempty=True, suppress:
0 files updated, 0 files merged, 0 files removed, 0 files unresolved

--- a/mercurial/hg.py	Thu Dec 17 11:00:06 2015 -0800
+++ b/mercurial/hg.py	Mon Dec 14 23:13:25 2015 +0000
@@ -637,7 +637,9 @@
             srcpeer.close()
     return srcpeer, destpeer
 
-def _showstats(repo, stats):
+def _showstats(repo, stats, quietempty=False):
+    if quietempty and not any(stats):
+        return
     repo.ui.status(_("%d files updated, %d files merged, "
                      "%d files removed, %d files unresolved\n") % stats)