There are several bot failures, some understood, some not. This will revert
bots to the old git until all the failures can be investigated.

Users who want the new git can still install it with an explicit:
set DEPOT_TOOLS_GIT_1852=1

R=hinoka@google.com

Review URL: https://codereview.chromium.org/150203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@250239 0039d316-1c4b-4281-b951-d872f2087c98