Update git fetch refspecs on git migrate

User may have set fetch only on the old default branch. This ensures
that fetch works with new default branch.

R=gavinmak@google.com

Bug: 1268775
Change-Id: I971cdd041e8e8a7f06a20f4de348c1395c7b6e06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3274108
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_migrate_default_branch.py

@@ -54,6 +54,12 @@ def main():
   if project_head != 'refs/heads/main':
     raise RuntimeError("The repository is not migrated yet.")
 
+  # User may have set to fetch only old default branch. Ensure fetch is tracking
+  # main too.
+  git_common.run('config', '--unset-all',
+                 'remote.origin.fetch', 'refs/heads/*')
+  git_common.run('config', '--add',
+                 'remote.origin.fetch', '+refs/heads/*:refs/remotes/origin/*')
   logging.info("Running fetch...")
   git_common.run('fetch', remote)
   logging.info("Updating remote HEAD...")

tests/git_migrate_default_branch_test.py

@@ -57,6 +57,8 @@ class CMDFormatTestCase(unittest.TestCase):
         return
       elif args[0] == 'branch':
         return
+      elif args[0] == 'config':
+        return
       raise Exception('Did not expect such run git command: %s' % args[0])
 
     mock_runs = mock.patch('git_migrate_default_branch.git_common.run',