Fix presubmit checks when --trybot-json flag is specified.

presubmit_support.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.
 
 """Enables directory-specific presubmit checks to run at upload and/or commit.
 """
 
 __version__ = '1.8.0'
 
@@ skipping 1002 matching lines @@
 
     Args:
       script_text: The text of the presubmit script.
       presubmit_path: Project script to run.
       project: Project name to pass to presubmit script for bot selection.
 
     Return:
       A list of try slaves.
     """
     context = {}
+    main_path = os.getcwd()
     try:
+      os.chdir(os.path.dirname(presubmit_path))
       exec script_text in context
     except Exception, e:
       raise PresubmitFailure('"%s" had an exception.\n%s' % (presubmit_path, e))
+    finally:
+      os.chdir(main_path)
 
     function_name = 'GetPreferredTrySlaves'
     if function_name in context:
       get_preferred_try_slaves = context[function_name]
       function_info = inspect.getargspec(get_preferred_try_slaves)
       if len(function_info[0]) == 1:
         result = get_preferred_try_slaves(project)
       elif len(function_info[0]) == 2:
         result = get_preferred_try_slaves(project, change)
       else:
@@ skipping 566 matching lines @@
   except PresubmitFailure, e:
     print >> sys.stderr, e
     print >> sys.stderr, 'Maybe your depot_tools is out of date?'
     print >> sys.stderr, 'If all fails, contact maruel@'
     return 2
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   sys.exit(Main(None))