[blinkpy] Set the default branch explicitly in git_unittest.py

Make the tests pass regardless of init.defaultBranch in gitconfig.

Bug: 1136164
Change-Id: I909c9e90144a621975982e3d8e32eb69fefdece9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2456407
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Auto-Submit: Robert Ma <robertma@chromium.org>
Reviewed-by: Rakib Hasan <rmhasan@google.com>
Cr-Commit-Position: refs/heads/master@{#815402}

third_party/blink/tools/blinkpy/common/checkout/git_unittest.py

@@ -27,6 +27,10 @@ class GitTestWithRealFilesystemAndExecutive(unittest.TestCase):
         self._run(['git', 'init', self.untracking_checkout_path])
 
         self._chdir(self.untracking_checkout_path)
+        # Explicitly create the default branch instead of relying on
+        # init.defaultBranch. We don't use the new --initial-branch flag with
+        # `git init` to keep the tests compatible with older versions of git.
+        self._run(['git', 'checkout', '-b', 'master'])
         self._set_user_config()
         self._write_text_file('foo_file', 'foo')
         self._run(['git', 'add', 'foo_file'])