merge-recursive: prepare for `merge_submodule()` to report errors

merge-recursive: prepare for `merge_submodule()` to report errors

The `merge_submodule()` function returns an integer that indicates
whether the merge was clean (returning 1) or unclean (returning 0).

Like the version in `merge-ort.c`, the version in `merge-recursive.c`
does not report any errors (such as repository corruption) by returning
-1 as of time of writing, even if the callers in `merge-ort.c` are
prepared for exactly such errors.

However, we want to teach (both variants of) the `merge_submodule()`
function that trick: to report errors by returning -1. Therefore,
prepare the caller in `merge-recursive.c` to handle that scenario.

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