build: Convert git_sha1_gen script to Python.

Python is the scripting language we've been using for scripts that need
to run across all supported platforms.

Shell is *not* a portable language for scripts.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

bin/git_sha1_gen.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python
+
+import os.path
+import subprocess
+import sys
+
+git_dir = os.path.join(os.path.dirname(sys.argv[0]), '..', '.git')
+try:
+    git_sha1 = subprocess.check_output([
+        'git',
+        '--git-dir=' + git_dir,
+        'rev-parse',
+        '--short=10',
+        'HEAD',
+    ], stderr=open(os.devnull, 'w'))
+except subprocess.CalledProcessError as e:
+    # don't print anything if git fails
+    pass
+else:
+    sys.stdout.write('#define MESA_GIT_SHA1 "git-%s"\n' % git_sha1.rstrip())