Make mark-merge-base cap the marked merge base at the real merge-base.

tests/git_common_test.py

 #!/usr/bin/env python
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Unit tests for git_common.py"""
 
 import binascii
 import collections
 import os
@@ skipping 418 matching lines @@
                      self.repo.run(self.gc.branch_config_map, 'base'))
 
     self.repo.run(self.gc.remove_merge_base, 'branch_K')
     self.repo.run(self.gc.remove_merge_base, 'branch_L')
 
     self.assertEqual(None,
                      self.repo.run(self.gc.config, 'branch.branch_K.base'))
 
     self.assertEqual({}, self.repo.run(self.gc.branch_config_map, 'base'))
 
+    # if it's too old, then it caps at merge-base
+    self.repo.run(self.gc.manual_merge_base, 'branch_K', self.repo['A'])
+
+    self.assertEqual(
+      self.repo['B'],
+      self.repo.run(self.gc.get_or_create_merge_base, 'branch_K', 'branch_G')
+    )
+
   def testGetBranchTree(self):
     skipped, tree = self.repo.run(self.gc.get_branch_tree)
     self.assertEqual(skipped, {'master', 'root_X'})
     self.assertEqual(tree, {
       'branch_G': 'root_A',
       'root_A': 'root_X',
       'branch_K': 'branch_G',
       'branch_L': 'branch_K',
       'branch_Z': 'root_X'
     })
@@ skipping 155 matching lines @@
 
       self.assertEquals(self.repo.git('status', '--porcelain').stdout, STATUS_1)
 
     self.repo.run(inner)
 
 
 if __name__ == '__main__':
   sys.exit(coverage_utils.covered_main(
     os.path.join(DEPOT_TOOLS_ROOT, 'git_common.py')
   ))