commit_id.py was change to try running 'git status' to confirm that git
was present.  This caused it to succeed if there was a git checkout
somewhere above ANGLE if ANGLE's .git didn't exist, incorrectly adding
a non-existant .git/HEAD file to the gn rule's inputs.

Always verify that ANGLE's .git/HEAD is present in commit_id.py.

BUG=angleproject:2344

Change-Id: I7e170f39c8d5cef73086ecf6d68925a1d3512de1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2082993
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>