py3: use .startswith() instead of bytes[0]

py3: use .startswith() instead of bytes[0]

bytes[0] returns the ascii value of character at 0 index.

Differential Revision: https://phab.mercurial-scm.org/D3580

--- a/mercurial/patch.py	Thu May 17 23:11:24 2018 -0700
+++ b/mercurial/patch.py	Sat May 19 00:19:56 2018 +0530
@@ -2492,14 +2492,14 @@
         chompline = line.rstrip('\n')
         # highlight tabs and trailing whitespace
         stripline = chompline.rstrip()
-        if line[0] == '-':
+        if line.startswith('-'):
             label = 'diff.deleted'
-        elif line[0] == '+':
+        elif line.startswith('+'):
             label = 'diff.inserted'
         else:
             raise error.ProgrammingError('unexpected hunk line: %s' % line)
         for token in tabsplitter.findall(stripline):
-            if '\t' == token[0]:
+            if token.startswith('\t'):
                 yield (token, 'diff.tab')
             else:
                 yield (token, label)