git: set GIT_EXEC_PATH

Some git commands are failing, because git --exec-path reports
non-reallocated path.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #478 from davidlt/git-set-GIT_EXEC_PATH

git: set GIT_EXEC_PATH
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable