git_sha1_gen: accept MESA_GIT_SHA1_OVERRIDE env var

git_sha1_gen: accept MESA_GIT_SHA1_OVERRIDE env var

If one uses a parent build script to download/build Mesa we may not
have a full git repository (maybe a tar archive) so the 'git rev-parse'
command will fail.

This updates the script to look for a MESA_GIT_SHA1_OVERRIDE env var.
If it's set, use that sha1 instead of using git rev-parse.  With this
change we can put a git hash in the GL_VERSION string even when we
don't have a git repo.

v2: incorporate Dylan's suggestions to simplify the code

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>