project: disable stat output when fast forwarding merges

Our sync output is pretty chatty, and the stat output on fast forward
merges doesn't really help.  Suppress it to tighten up the output.

Change-Id: I91e50639b3cd8db9df3d13a7da6d1aaa70d7932f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255412
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -2584,7 +2584,7 @@ class Project(object):
       raise GitError('%s rebase %s ' % (self.name, upstream))
 
   def _FastForward(self, head, ffonly=False):
-    cmd = ['merge', head]
+    cmd = ['merge', '--no-stat', head]
     if ffonly:
       cmd.append("--ff-only")
     if GitCommand(self, cmd).Wait() != 0: