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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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