apply: only pass required data to skip_tree_prefix

apply: only pass required data to skip_tree_prefix

Currently the 'skip_tree_prefix()' function takes 'struct apply_state'
as parameter, even though it only needs the p_value 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>