The solution is the same as in GYP version:
1. Check if .git/index is available in angle directory.
2. Use generated commit.h file if git is available or copy the default
file instead.

Change-Id: Ic3cedf6793c368070228a63c0eb270ca6fb47f81
Reviewed-on: https://chromium-review.googlesource.com/289391Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Tomasz Moniuszko <tmoniuszko@opera.com>