test-convert-git: silence commit output for stablity

When running the tests with 1.7.7.6, I get 'files' and 'insertions' instead of
the singular forms, and there is also an additional '0 deletions(-)' at the end.
Since this doesn't seem important to the test, silence it.

tests/test-convert-git.t

@@ -689,10 +689,7 b' Run convert when the remote branches hav'
 
   $ cd git-repo7
   $ echo a >> a
-  $ git commit -am "move master forward"
-  [master 0c81947] move master forward
-   Author: nottest <[e-mail]>
-   1 file changed, 1 insertion(+)
+  $ git commit -q -am "move master forward"
   $ cd ..
   $ rm -rf hg-repo7
   $ hg convert --config convert.git.remoteprefix=origin git-repo7-client hg-repo7