contint: jenkins git config core.packedGitLimit=2G

Whenever running `git gc --aggressive` on gallium as user jenkins, we
end up sending the machine to swap.  The reason is that when reading
pack files, git map them in memory up to 8GB on 64 bits platforms.

2GB sounds sane enough.

bug: 56717
Change-Id: Id5aa9a853a3fd100437b2b3db459d177f05bc9ab
Merge "contint: jenkins git config core.packedGitLimit=2G" into production