Clear configured merge-base when switching upstream.

git_cl.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 from distutils.version import LooseVersion
@@ skipping 2297 matching lines @@
 @subcommand.usage('[new upstream branch]')
 def CMDupstream(parser, args):
   """Prints or sets the name of the upstream branch, if any."""
   _, args = parser.parse_args(args)
   if len(args) > 1:
     parser.error('Unrecognized args: %s' % ' '.join(args))
 
   cl = Changelist()
   if args:
     # One arg means set upstream branch.
-    RunGit(['branch', '--set-upstream', cl.GetBranch(), args[0]])
+    branch = cl.GetBranch()
+    RunGit(['branch', '--set-upstream', branch, args[0]])
     cl = Changelist()
     print "Upstream branch set to " + cl.GetUpstreamBranch()
+
+    # Clear configured merge-base, if there is one.
+    git_common.remove_merge_base(branch)
   else:
     print cl.GetUpstreamBranch()
   return 0
 
 
 def CMDweb(parser, args):
   """Opens the current CL in the web browser."""
   _, args = parser.parse_args(args)
   if args:
     parser.error('Unrecognized args: %s' % ' '.join(args))
@@ skipping 226 matching lines @@
         ('AppEngine is misbehaving and returned HTTP %d, again. Keep faith '
           'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
 
 
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   sys.exit(main(sys.argv[1:]))