merge-recursive: switch to returning errors instead of dying

merge-recursive: switch to returning errors instead of dying

The recursive merge machinery is supposed to be a library function, i.e.
it should return an error when it fails. Originally the functions were
part of the builtin "merge-recursive", though, where it was simpler to
call die() and be done with error handling.

The existing callers were already prepared to detect negative return
values to indicate errors and to behave as previously: exit with code 128
(which is the same thing that die() does, after printing the message).

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