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>

Documentation/git-apply.txt

@@ -9,7 +9,8 @@ git-apply - Apply a patch to files and/or to the index
 SYNOPSIS
 --------
 [verse]
-'git apply' [--stat] [--numstat] [--summary] [--check] [--index | --intent-to-add] [--3way]
+'git apply' [--stat] [--numstat] [--summary] [--check]
+	  [--index | --intent-to-add] [--3way] [--ours | --theirs | --union]
 	  [--apply] [--no-add] [--build-fake-ancestor=<file>] [-R | --reverse]
 	  [--allow-binary-replacement | --binary] [--reject] [-z]
 	  [-p<n>] [-C<n>] [--inaccurate-eof] [--recount] [--cached]
@@ -92,6 +93,12 @@ OPTIONS
 	When used with the `--cached` option, any conflicts are left at higher stages
 	in the cache.
 
+--ours::
+--theirs::
+--union::
+	Instead of leaving conflicts in the file, resolve conflicts favouring
+	our (or their or both) side of the lines. Requires --3way.
+
 --build-fake-ancestor=<file>::
 	Newer 'git diff' output has embedded 'index information'
 	for each blob to help identify the original version that

apply.c

@@ -3561,6 +3561,7 @@ static int three_way_merge(struct apply_state *state,
 			   const struct object_id *theirs)
 {
 	mmfile_t base_file, our_file, their_file;
+	struct ll_merge_options merge_opts = LL_MERGE_OPTIONS_INIT;
 	mmbuffer_t result = { NULL };
 	enum ll_merge_result status;
 
@@ -3573,12 +3574,13 @@ static int three_way_merge(struct apply_state *state,
 	read_mmblob(&base_file, base);
 	read_mmblob(&our_file, ours);
 	read_mmblob(&their_file, theirs);
+	merge_opts.variant = state->merge_variant;
 	status = ll_merge(&result, path,
 			  &base_file, "base",
 			  &our_file, "ours",
 			  &their_file, "theirs",
 			  state->repo->index,
-			  NULL);
+			  &merge_opts);
 	if (status == LL_MERGE_BINARY_CONFLICT)
 		warning("Cannot merge binary files: %s (%s vs. %s)",
 			path, "ours", "theirs");
@@ -5151,6 +5153,15 @@ int apply_parse_options(int argc, const char **argv,
 			N_("also apply the patch (use with --stat/--summary/--check)")),
 		OPT_BOOL('3', "3way", &state->threeway,
 			 N_( "attempt three-way merge, fall back on normal patch if that fails")),
+		OPT_SET_INT_F(0, "ours", &state->merge_variant,
+			N_("for conflicts, use our version"),
+			XDL_MERGE_FAVOR_OURS, PARSE_OPT_NONEG),
+		OPT_SET_INT_F(0, "theirs", &state->merge_variant,
+			N_("for conflicts, use their version"),
+			XDL_MERGE_FAVOR_THEIRS, PARSE_OPT_NONEG),
+		OPT_SET_INT_F(0, "union", &state->merge_variant,
+			N_("for conflicts, use a union version"),
+			XDL_MERGE_FAVOR_UNION, PARSE_OPT_NONEG),
 		OPT_FILENAME(0, "build-fake-ancestor", &state->fake_ancestor,
 			N_("build a temporary index based on embedded index information")),
 		/* Think twice before adding "--nul" synonym to this */
@@ -5190,5 +5201,10 @@ int apply_parse_options(int argc, const char **argv,
 		OPT_END()
 	};
 
-	return parse_options(argc, argv, state->prefix, builtin_apply_options, apply_usage, 0);
+	argc = parse_options(argc, argv, state->prefix, builtin_apply_options, apply_usage, 0);
+
+	if (state->merge_variant && !state->threeway)
+		die(_("--ours, --theirs, and --union require --3way"));
+
+	return argc;
 }

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;

t/t4108-apply-threeway.sh

@@ -82,6 +82,46 @@ test_expect_success 'apply with --3way with merge.conflictStyle = diff3' '
 	test_apply_with_3way
 '
 
+test_apply_with_3way_favoritism () {
+	apply_arg=$1
+	merge_arg=$2
+
+	# Merging side should be similar to applying this patch
+	git diff ...side >P.diff &&
+
+	# The corresponding conflicted merge
+	git reset --hard &&
+	git checkout main^0 &&
+	git merge --no-commit $merge_arg side &&
+	git ls-files -s >expect.ls &&
+	print_sanitized_conflicted_diff >expect.diff &&
+
+	# should apply successfully
+	git reset --hard &&
+	git checkout main^0 &&
+	git apply --index --3way $apply_arg P.diff &&
+	git ls-files -s >actual.ls &&
+	print_sanitized_conflicted_diff >actual.diff &&
+
+	# The result should resemble the corresponding merge
+	test_cmp expect.ls actual.ls &&
+	test_cmp expect.diff actual.diff
+}
+
+test_expect_success 'apply with --3way --ours' '
+	test_apply_with_3way_favoritism --ours -Xours
+'
+
+test_expect_success 'apply with --3way --theirs' '
+	test_apply_with_3way_favoritism --theirs -Xtheirs
+'
+
+test_expect_success 'apply with --3way --union' '
+	echo "* merge=union" >.gitattributes &&
+	test_apply_with_3way_favoritism --union &&
+	rm .gitattributes
+'
+
 test_expect_success 'apply with --3way with rerere enabled' '
 	test_config rerere.enabled true &&