builtin-apply.c: push match-beginning/end logic down

builtin-apply.c: push match-beginning/end logic down

This moves the logic to force match at the beginning and/or at
the end of the buffer to the actual function that finds the
match from its caller.  This is a necessary preparation for the
next step to allow matching disregarding certain differences,
such as whitespace changes.

We probably could optimize this even more by taking advantage of
the fact that match_beginning and match_end forces the match to
be at an exact location (anchored at the beginning and/or the
end), but that's for another commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>