Exclude .md files from version control conflict detection.

PRESUBMIT.py

 # 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 823 matching lines @@
   if warnings:
     results.append(output_api.PresubmitPromptOrNotify(_INCLUDE_ORDER_WARNING,
                                                       warnings))
   return results
 
 
 def _CheckForVersionControlConflictsInFile(input_api, f):
   pattern = input_api.re.compile('^(?:<<<<<<<|>>>>>>>) |^=======$')
   errors = []
   for line_num, line in f.ChangedContents():
+    if f.LocalPath().endswith('.md'):
+      # First-level headers in markdown look a lot like version control
+      # conflict markers. http://daringfireball.net/projects/markdown/basics
+      continue
     if pattern.match(line):
       errors.append('    %s:%d %s' % (f.LocalPath(), line_num, line))
   return errors
 
 
 def _CheckForVersionControlConflicts(input_api, output_api):
   """Usually this is not intentional and will cause a compile failure."""
   errors = []
   for f in input_api.AffectedFiles():
     errors.extend(_CheckForVersionControlConflictsInFile(input_api, f))
@@ skipping 949 matching lines @@
     # Explicitly iterate over copies of dicts since we mutate them.
     for master in builders.keys():
       for builder in builders[master].keys():
         # Do not trigger presubmit builders, since they're likely to fail
         # (e.g. OWNERS checks before finished code review), and we're
         # running local presubmit anyway.
         if 'presubmit' in builder:
           builders[master].pop(builder)
 
   return builders