apply: support --ours, --theirs, and --union for three-way merges

apply: support --ours, --theirs, and --union for three-way merges

--ours, --theirs, and --union are already supported in `git merge-file`
for automatically resolving conflicts in favor of one version or the
other, instead of leaving conflict markers in the file. Support them in
`git apply -3` as well because the two commands do the same kind of
file-level merges.

In case in the future --ours, --theirs, and --union gain a meaning
outside of three-way-merges, they do not imply --3way but rather must be
specified alongside it.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>