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

Doing bytes[0] will return the ascii value of that position which breaks
comparison with a bytechar.

This makes test-absorb.t work again on py3.

Differential Revision: https://phab.mercurial-scm.org/D6508

hgext/absorb.py

@@ -871,7 +871,7 def _parsechunk(hunk):
     patchlines = mdiff.splitnewlines(buf.getvalue())
     # hunk.prettystr() will update hunk.removed
     a2 = a1 + hunk.removed
-    blines = [l[1:] for l in patchlines[1:] if l[0] != '-']
+    blines = [l[1:] for l in patchlines[1:] if not l.startswith('-')]
     return path, (a1, a2, blines)
 
 def overlaydiffcontext(ctx, chunks):