Get the real commit hash, not the bundle hash

It's extracted from the commit message of the bundle.

python/servo/command_base.py

@@ -734,9 +734,16 @@ def build_env(self, hosts_file_path=None, target=None, is_build=False, test_unit
 
         git_info = []
         if os.path.isdir('.git') and is_build:
-            git_sha = subprocess.check_output([
-                'git', 'rev-parse', '--short', 'HEAD'
+            # Get the subject of the bundle commit
+            git_bundle_subject = subprocess.check_output([
+                'git', 'show', '-s', '--format=%s', 'HEAD'
             ]).strip()
+            # Get the SHA-1 from the bundle subject: "Shallow version of commit {sha1}"
+            git_sha = git_bundle_subject.split(' ')[-1]
+            # Verify that it's a valid commit
+            # NOTE: this will pass even if `git_sha` is 'master' or another branch or ref
+            subprocess.check_call('git', 'cat-file', 'commit', git_sha)
+
             git_is_dirty = bool(subprocess.check_output([
                 'git', 'status', '--porcelain'
             ]).strip())