spec: check git_dir first
spec: check git_dir first

`wip?` depends on git working, so the current behavior goes :boom: for non-
plugin GERRIT_PROJECTs

Change-Id: I43fc95125add6afeb35c931305a60ffb2e754509
Reviewed-on: https://gerrit.instructure.com/109929
Tested-by: Jenkins
Reviewed-by: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
Product-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
QA-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,instructure/canvas-lms