builtin/apply: move 'fn_table' global into 'struct apply_state'
To libify the apply functionality the 'fn_table' variable should
not be static and global to the file. Let's move it into
'struct apply_state'.
As fn_table is cleared at the end of apply_patch(), it is not
necessary to clear it in clear_apply_state().
Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>