If the stderr buffer used in git diff became full
before stdout was completely consumed, git cl upload
would deadlock.

This also sets GIT_PAGER to be perfectly sure no pager
is triggered by the git commands run. I haven't
seen that as a problem but it's a reasonable (and
common) precaution.

BUG=558726

Review URL: https://codereview.chromium.org/1544813002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298090 0039d316-1c4b-4281-b951-d872f2087c98