Add better error messages to git_reparent_branch.

tests/git_rebase_update_test.py

Index: tests/git_rebase_update_test.py
diff --git a/tests/git_rebase_update_test.py b/tests/git_rebase_update_test.py
index 976d9a6d0803d98852a32a6df96a9754e68078ad..e8d7d5f9029479cfd216b472dca2b1f84af3a38d 100755
--- a/tests/git_rebase_update_test.py
+++ b/tests/git_rebase_update_test.py
@@ -387,6 +387,14 @@ class GitRebaseUpdateTest(git_test_utils.GitRepoReadWriteTestBase):
                   M H I J K L
     """)
 
+  def testReparentBranchWithoutUpstream(self):
+    self.repo.git('branch', 'nerp')
+    self.repo.git('checkout', 'nerp')
+
+    _, err = self.repo.capture_stdio(self.rp.main, ['branch_K'])
+
+    self.assertIn('Unable to determine nerp@{upstream}', err)
+
 
 if __name__ == '__main__':
   sys.exit(coverage_utils.covered_main((