apply: only pass required data to check_header_line

apply: only pass required data to check_header_line

Currently the 'check_header_line()' function takes 'struct
apply_state' as parameter, even though it only needs the linenr from
that struct.

This function is in the callchain of 'parse_git_header()', which we
want to make more generally useful in a subsequent commit.  To make
that happen we only want to pass in the required data to
'parse_git_header()', and not the whole 'struct apply_state', and thus
we want functions in the callchain of 'parse_git_header()' to only
take arguments they really need.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>