py3: slice on bytes instead of indexing

Indexing returns the ascii value on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D1793

mercurial/mdiff.py

@@ -356,7 +356,7 b' def _unidiff(t1, t2, opts=defaultopts):'
             # the previous hunk context until we find a line starting with an
             # alphanumeric char.
             for i in xrange(astart - 1, lastpos - 1, -1):
-                if l1[i][0].isalnum():
+                if l1[i][0:1].isalnum():
                     func = ' ' + l1[i].rstrip()[:40]
                     lastfunc[1] = func
                     break