buildsys: move file regeneration logic to the script itself

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

bin/git_sha1_gen.py

@@ -6,6 +6,7 @@ The output of this script goes to stdout.
 """
 
 
+import argparse
 import os
 import os.path
 import subprocess
@@ -27,10 +28,20 @@ def get_git_sha1():
         git_sha1 = ''
     return git_sha1
 
+parser = argparse.ArgumentParser()
+parser.add_argument('--output', help='File to write the #define in',
+        required=True)
+args = parser.parse_args()
 
 git_sha1 = os.environ.get('MESA_GIT_SHA1_OVERRIDE', get_git_sha1())[:10]
 if git_sha1:
     git_sha1_h_in_path = os.path.join(os.path.dirname(sys.argv[0]),
             '..', 'src', 'git_sha1.h.in')
     with open(git_sha1_h_in_path , 'r') as git_sha1_h_in:
-        sys.stdout.write(git_sha1_h_in.read().replace('@VCS_TAG@', git_sha1))
+        new_sha1 = git_sha1_h_in.read().replace('@VCS_TAG@', git_sha1)
+        if os.path.isfile(args.output):
+            with open(args.output, 'r') as git_sha1_h:
+                if git_sha1_h.read() == new_sha1:
+                    quit()
+        with open(args.output, 'w') as git_sha1_h:
+            git_sha1_h.write(new_sha1)