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>

apply.h

@@ -59,6 +59,7 @@ struct apply_state {
 	struct repository *repo;
 	const char *index_file;
 	enum apply_verbosity apply_verbosity;
+	int merge_variant;
 	char *fake_ancestor;
 	const char *patch_input_file;
 	int line_termination;