merge-recursive: move some definitions around to clean up the header

merge-recursive: move some definitions around to clean up the header

No substantive code changes (view this with diff --color-moved), but
a few small code cleanups:
  * Move structs and an inline function only used by merge-recursive.c
    into merge-recursive.c
  * Re-order function declarations to be more logical
  * Add or fix some explanatory comments

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>