Now update git depot_tools checkouts, also add automatic update for git-cl-repo and git-try-repo.

The difference with 18633 is that it leaves bootstrap/gclient.sh along. I'll remove it in a month or so to not break anyone in the meantime.

TEST=none
BUG=none
TBR=evan
Review URL: http://codereview.chromium.org/129001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@18637 0039d316-1c4b-4281-b951-d872f2087c98