Return an error from unpack_trees() instead of calling die(), and exit
with an error in read-tree, builtin-commit, and diff-lib. merge-recursive
already expected an error return from unpack_trees, so it doesn't need to
be changed. The merge function can return negative to abort.
This will be used in builtin-checkout -m.
Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>