git-apply--interactive: Refactor patch mode code

git-apply--interactive: Refactor patch mode code

This makes some aspects of the 'git add -p' loop configurable (within
the code), so that we can later reuse git-add--interactive for other
similar tools.

Most fields are fairly straightforward, but APPLY gets a subroutine
(instead of just a string a la 'apply --cached') so that we can handle
'checkout -p', which will need to atomically apply the patch twice
(index and worktree).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>