Revert of Specify GIT_DIR when running git-config in a mirror. (patchset #4 id:60001 of https://codereview.chromium.org/1167193002/)

Reason for revert:
Potentially broke the NaCl toolchain builders:
crbug.com/498942

Speculative revert.

Original issue's description:
> Specify GIT_DIR when running git-config in a mirror.
> 
> BUG=497894
> R=hinoka@chromium.org,agable@chromium.org
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=295587

TBR=agable@chromium.org,szager@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=497894

Review URL: https://codereview.chromium.org/1179593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295607 0039d316-1c4b-4281-b951-d872f2087c98