Use git_common to call git

In preparation for some Windows optimizations to how git_common calls
git it is important to use git_common more widely, specifically from
scm.py and gclient_scm.py. This change updates scm.py and gclient_scm.py
and updates the associated tests:

Test command lines used when updating the tests include:
vpython3 tests/gclient_scm_test.py ManagedGitWrapperTestCaseMock.testUpdateConflict

vpython3 tests/gclient_scm_test.py GerritChangesTest.testRecoversAfterPatchFailure

vpython3 tests/gerrit_util_test.py CookiesAuthenticatorTest.testGetGitcookiesPath

Bug: 332982922
Change-Id: I7aacb110b2888c164259815385cd77e26942adc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5478509
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

gclient_scm.py

@@ -22,6 +22,7 @@ import traceback
 import gclient_utils
 import gerrit_util
 import git_cache
+import git_common
 import scm
 import subprocess2
 
@@ -1578,8 +1579,8 @@ class GitWrapper(SCMWrapper):
             env.setdefault(
                 'GIT_DIR',
                 os.path.abspath(os.path.join(self.checkout_path, '.git')))
-        ret = subprocess2.check_output(['git'] + args, env=env,
-                                       **kwargs).decode('utf-8')
+        kwargs.setdefault('env', env)
+        ret = git_common.run(*args, **kwargs)
         if strip:
             ret = ret.strip()
         self.Print('Finished running: %s %s' % ('git', ' '.join(args)))