Add unit tests for guessing main as upstream

R=ehmaldonado@chromium.org

Bug: 1143724
Change-Id: I69ce296a27b36c1073999f2b3c51421ac94e7ef2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2519254
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

tests/scm_unittest.py

@@ -185,6 +185,12 @@ class RealGitTest(fake_repos.FakeReposTestBase):
     self.assertEqual(
         ('origin', 'refs/heads/master'), scm.GIT.FetchUpstreamTuple(self.cwd))
 
+  @mock.patch('scm.GIT.GetRemoteBranches',
+              return_value=['origin/master', 'origin/main'])
+  def testFetchUpstreamTuple_GuessOriginMain(self, _mockGetRemoteBranches):
+    self.assertEqual(('origin', 'refs/heads/main'),
+                     scm.GIT.FetchUpstreamTuple(self.cwd))
+
   def testFetchUpstreamTuple_RietveldUpstreamConfig(self):
     scm.GIT.SetConfig(self.cwd, 'rietveld.upstream-branch', 'rietveld-upstream')
     scm.GIT.SetConfig(self.cwd, 'rietveld.upstream-remote', 'rietveld-remote')