Auto merge of #11262 - campaul:add_version_flag, r=Manishearth

Add `--version` flag

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy --faster` does not report any errors
- [X] These changes fix #11241 (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

Not 100% sure of a good way to test this, so I'm submitting as is for feedback. Manually testing it appears to work fine.

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python/servo/command_base.py

@@ -374,6 +374,22 @@ class CommandBase(object):
 
         env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -W unused-extern-crates"
 
+        git_info = []
+        if os.path.isdir('.git'):
+            git_sha = subprocess.check_output([
+                'git', 'rev-parse', '--short', 'HEAD'
+            ]).strip()
+            git_is_dirty = bool(subprocess.check_output([
+                'git', 'status', '--porcelain'
+            ]).strip())
+
+            git_info.append('')
+            git_info.append(git_sha)
+            if git_is_dirty:
+                git_info.append('dirty')
+
+        env['GIT_INFO'] = '-'.join(git_info)
+
         return env
 
     def servo_crate(self):