builtin/apply: add 'lock_file' pointer into 'struct apply_state'

builtin/apply: add 'lock_file' pointer into 'struct apply_state'

We cannot have a 'struct lock_file' allocated on the stack, as lockfile.c
keeps a linked list of all created lock_file structures.

Also 'struct apply_state' users might later want the same 'struct lock_file'
instance to be reused by different series of calls to the apply api.

So let's add a 'struct lock_file *lock_file' pointer into 'struct apply_state'
and have the user of 'struct apply_state' allocate memory for the actual
'struct lock_file' instance.

Let's also add an argument to init_apply_state(), so that the caller can
easily supply a pointer to the allocated instance.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>