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
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1