Diffusion - add verifyGitOrigin check to git fetch operation

Summary: Fixes T4946. Theoretically.

Test Plan:

also unit tests.

also

cd /var/repo/X
git remote remove origin # simulates origin-missing clone under 1.7.1
cd /path/to/phabricator
./bin/repository pull X

and observed no errors

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4946, T5938

Differential Revision: https://secure.phabricator.com/D10855