Turns out 'git gc' emits text even when -q is specified.  /dev/null time...

Test Plan:
None.  I may need to revisit this again if it turns out 'git gc' emits
to stderr in normal operation.  Hopefully it doesn't...

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D1040