Exclude .md files from version control conflict detection.

Turns out top-level headers basically look exactly like
a merge conflict marker.

R=jlklein@chromium.org
BUG=495338

Review URL: https://codereview.chromium.org/1153013005

Cr-Commit-Position: refs/heads/master@{#332313}

PRESUBMIT.py

@@ -841,6 +841,10 @@ 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

PRESUBMIT_test.py

@@ -270,6 +270,16 @@ class VersionControlConflictsTest(unittest.TestCase):
     self.assertTrue('3' in errors[1])
     self.assertTrue('5' in errors[2])
 
+  def testIgnoresReadmes(self):
+    lines = ['A First Level Header',
+             '====================',
+             '',
+             'A Second Level Header',
+             '---------------------']
+    errors = PRESUBMIT._CheckForVersionControlConflictsInFile(
+        MockInputApi(), MockFile('some/polymer/README.md', lines))
+    self.assertEqual(0, len(errors))
+
 class UmaHistogramChangeMatchedOrNotTest(unittest.TestCase):
   def testTypicalCorrectlyMatchedChange(self):
     diff_cc = ['UMA_HISTOGRAM_BOOL("Bla.Foo.Dummy", true)']