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
revset: fix merge() to fall back to changectx API if wdir specified
I have a code which basically runs "0:wdir() & <user-revset>", and it crashed
if merge() were passed in.