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>

bin/git_sha1_gen.py

@@ -1,20 +1,33 @@
 #!/usr/bin/env python
 
+"""
+Generate the contents of the git_sha1.h file.
+The output of this script goes to stdout.
+"""
+
+
+import os
 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')).decode("ascii")
-except:
-    # don't print anything if it fails
-    pass
-else:
+
+def get_git_sha1():
+    """Try to get the git SHA1 with git rev-parse."""
+    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',
+            'HEAD',
+        ], stderr=open(os.devnull, 'w')).decode("ascii")
+    except:
+        # don't print anything if it fails
+        git_sha1 = ''
+    return git_sha1
+
+
+git_sha1 = os.environ.get('MESA_GIT_SHA1_OVERRIDE', get_git_sha1())[:10]
+if git_sha1:
     sys.stdout.write('#define MESA_GIT_SHA1 "git-%s"\n' % git_sha1.rstrip())