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>

scm.py

@@ -10,6 +10,7 @@ import re
 from typing import Mapping, List
 
 import gclient_utils
+import git_common
 import subprocess2
 
 # TODO: Should fix these warnings.
@@ -105,14 +106,10 @@ class GIT(object):
 
     @staticmethod
     def Capture(args, cwd=None, strip_out=True, **kwargs):
-        env = GIT.ApplyEnvVars(kwargs)
-        output = subprocess2.check_output(['git'] + args,
-                                          cwd=cwd,
-                                          stderr=subprocess2.PIPE,
-                                          env=env,
-                                          **kwargs)
-        output = output.decode('utf-8', 'replace')
-        return output.strip() if strip_out else output
+        kwargs.setdefault('env', GIT.ApplyEnvVars(kwargs))
+        kwargs.setdefault('cwd', cwd)
+        kwargs.setdefault('autostrip', strip_out)
+        return git_common.run(*args, **kwargs)
 
     @staticmethod
     def CaptureStatus(cwd,