builtin/apply: move 'state' init into init_apply_state()

builtin/apply: move 'state' init into init_apply_state()

When the apply functionality will be libified, the 'struct apply_state'
will be used by different pieces of code.

To properly initialize a 'struct apply_state', let's provide a nice
and easy to use init_apply_state() function.

Let's also provide clear_apply_state() to release memory used by
'struct apply_state' members, so that a 'struct apply_state' instance
can be easily reused without leaking memory.

Note that clear_apply_state() does nothing for now, but it will later.

While at it, let's rename 'prefix_' parameter to 'prefix'.

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