Auto merge of #12752 - UK992:mach, r=metajack

Run git commands only with mach build

This should solve problem with slow run on Windows mention in https://github.com/servo/servo/issues/12730, which is cause by slow git.

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python/servo/build_commands.py

@@ -209,7 +209,7 @@ class MachCommands(CommandBase):
             opts += ["--features", "%s" % ' '.join(features)]
 
         build_start = time()
-        env = self.build_env(target=target)
+        env = self.build_env(target=target, is_build=True)
 
         if android:
             # Build OpenSSL for android
@@ -299,7 +299,7 @@ class MachCommands(CommandBase):
         build_start = time()
         with cd(path.join("ports", "cef")):
             ret = call(["cargo", "build"] + opts,
-                       env=self.build_env(), verbose=verbose)
+                       env=self.build_env(is_build=True), verbose=verbose)
         elapsed = time() - build_start
 
         # Generate Desktop Notification if elapsed-time > some threshold value
@@ -334,7 +334,7 @@ class MachCommands(CommandBase):
         if release:
             opts += ["--release"]
 
-        env = self.build_env()
+        env = self.build_env(is_build=True)
         env["CARGO_TARGET_DIR"] = path.join(self.context.topdir, "target", "geckolib").encode("UTF-8")
 
         build_start = time()

python/servo/command_base.py

@@ -348,7 +348,7 @@ class CommandBase(object):
                                   " --release" if release else ""))
         sys.exit()
 
-    def build_env(self, hosts_file_path=None, target=None):
+    def build_env(self, hosts_file_path=None, target=None, is_build=False):
         """Return an extended environment dictionary."""
         env = os.environ.copy()
         if sys.platform == "win32" and type(env['PATH']) == unicode:
@@ -445,7 +445,7 @@ class CommandBase(object):
         env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -W unused-extern-crates"
 
         git_info = []
-        if os.path.isdir('.git'):
+        if os.path.isdir('.git') and is_build:
             git_sha = subprocess.check_output([
                 'git', 'rev-parse', '--short', 'HEAD'
             ]).strip()