Add some pack options to make sure 'git gc' doesn't cause swapping.

When we gc the i18n repos, git gc will take a lot of memory because of
the big files they have.  These options should help with that,
according to:
   http://stackoverflow.com/questions/3095737/is-there-a-way-to-limit-the-amount-of-memory-that-git-gc-uses

Auditors: chris

Test plan:
Edited jenkins .gitconfig manually and ran 'git gc' in
   /mnt/jenkins/jobs/update-translations/workspace/webapp-i18n-data
and ran top and saw it using 2.5G of virtual memory and <500M of RSS.