merge-recursive: avoid returning a wholesale struct

merge-recursive: avoid returning a wholesale struct

It is technically allowed, as per C89, for functions' return type to
be complete structs (i.e. *not* just pointers to structs).

However, it was just an oversight of this developer when converting
Python code to C code in 6d297f8 (Status update on merge-recursive in
C, 2006-07-08) which introduced such a return type.

Besides, by converting this construct to pass in the struct, we can now
start returning a value that can indicate errors in future patches. This
will help the current effort to libify merge-recursive.c.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>