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
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors