range-diff: add --matched-only to skip one-sided commits

Reviewing a range-diff often means scrolling past commits that were
simply added or dropped, when only the ones that correspond between
the two ranges are of interest.

--left-only and --right-only already each suppress one of those
one-sided groups, but they are defined as "only show this side" and
so cannot be given together, which is exactly why show_range_diff()
already rejected that combination. Give the "show only the commits
that correspond on both sides" behavior its own name, --matched-only,
instead of asking users to reach for a combination that errors out.

Extend the existing '--left-only'/'--right-only' conflict check to
also reject any combination with --matched-only, since all three
narrow the output in ways that cannot be combined.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-range-diff.adoc

@@ -10,7 +10,8 @@ SYNOPSIS
 [synopsis]
 git range-diff [--color=[<when>]] [--no-color] [<diff-options>]
 	[--no-dual-color] [--creation-factor=<factor>]
-	[--left-only | --right-only] [--diff-merges=<format>]
+	[--left-only | --right-only | --matched-only]
+	[--diff-merges=<format>]
 	[--remerge-diff] [--no-notes | --notes[=<ref>]]
 	( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )
 	[[--] <path>...]
@@ -82,6 +83,12 @@ to revert to color all lines according to the outer diff markers
 	Suppress commits that are missing from the second specified range
 	(or the "right range" when using the `<rev1>...<rev2>` form).
 
+`--matched-only`::
+	Only emit commits that have a corresponding commit in the other
+	range, suppressing any commit that exists on only one side. Useful
+	to skip added or removed commits when reviewing how the commits
+	that survived a rebase changed.
+
 `--diff-merges=<format>`::
 	Instead of ignoring merge commits, generate diffs for them using the
 	corresponding `--diff-merges=<format>` option of linkgit:git-log[1],

builtin/range-diff.c

@@ -46,7 +46,7 @@ int cmd_range_diff(int argc,
 		.diffopt = &diffopt,
 		.log_arg = &log_arg
 	};
-	int simple_color = -1, left_only = 0, right_only = 0;
+	int simple_color = -1, left_only = 0, right_only = 0, matched_only = 0;
 	struct option range_diff_options[] = {
 		OPT_INTEGER(0, "creation-factor",
 			    &range_diff_opts.creation_factor,
@@ -68,6 +68,8 @@ int cmd_range_diff(int argc,
 			 N_("only emit output related to the first range")),
 		OPT_BOOL(0, "right-only", &right_only,
 			 N_("only emit output related to the second range")),
+		OPT_BOOL(0, "matched-only", &matched_only,
+			 N_("only emit commits that have a corresponding commit in the other range")),
 		OPT_END()
 	};
 	struct option *options;
@@ -186,6 +188,7 @@ int cmd_range_diff(int argc,
 	range_diff_opts.dual_color = simple_color < 1;
 	range_diff_opts.left_only = left_only;
 	range_diff_opts.right_only = right_only;
+	range_diff_opts.matched_only = matched_only;
 	res = show_range_diff(range1.buf, range2.buf, &range_diff_opts);
 
 	strvec_clear(&log_arg);

range-diff.c

@@ -591,8 +591,15 @@ int show_range_diff(const char *range1, const char *range2,
 	struct string_list branch2 = STRING_LIST_INIT_DUP;
 	unsigned int include_merges = range_diff_opts->include_merges;
 
-	if (range_diff_opts->left_only && range_diff_opts->right_only)
-		res = error(_("options '%s' and '%s' cannot be used together"), "--left-only", "--right-only");
+	if (range_diff_opts->left_only + range_diff_opts->right_only +
+	    range_diff_opts->matched_only > 1)
+		res = error(_("options '%s', '%s', or '%s' cannot be used together"),
+			    "--left-only", "--right-only", "--matched-only");
+
+	if (range_diff_opts->matched_only) {
+		range_diff_opts->left_only = 1;
+		range_diff_opts->right_only = 1;
+	}
 
 	if (!res && read_patches(range1, &branch1, range_diff_opts->log_arg, include_merges))
 		res = error(_("could not parse log for '%s'"), range1);

range-diff.h

@@ -19,7 +19,7 @@
 struct range_diff_options {
 	int creation_factor;
 	unsigned dual_color:1;
-	unsigned left_only:1, right_only:1;
+	unsigned left_only:1, right_only:1, matched_only:1;
 	unsigned include_merges:1;
 	size_t max_memory;
 	const struct diff_options *diffopt; /* may be NULL */

t/t3206-range-diff.sh

@@ -860,6 +860,69 @@ test_expect_success '--left-only/--right-only' '
 	test_cmp expect actual
 '
 
+test_expect_success '--left-only, --right-only and --matched-only are incompatible' '
+	test_must_fail git range-diff --left-only --right-only ...common 2>err &&
+	test_grep "cannot be used together" err &&
+
+	test_must_fail git range-diff --left-only --matched-only ...common 2>err &&
+	test_grep "cannot be used together" err &&
+
+	test_must_fail git range-diff --right-only --matched-only ...common 2>err &&
+	test_grep "cannot be used together" err &&
+
+	test_must_fail git range-diff --left-only --right-only --matched-only \
+		...common 2>err &&
+	test_grep "cannot be used together" err
+'
+
+test_expect_success '--left-only, --right-only and --matched-only each suppress one-sided commits' '
+	test_create_repo matched-only &&
+	(
+		cd matched-only &&
+		git switch --orphan combined-old &&
+		test_commit c-first &&
+		test_commit c-old-only &&
+		test_commit c-common &&
+		git switch -C combined-new c-first &&
+		test_commit c-new-only &&
+		git cherry-pick c-common &&
+
+		old_only_oid=$(git rev-parse --short=7 c-old-only) &&
+		new_only_oid=$(git rev-parse --short=7 c-new-only) &&
+		common_old_oid=$(git rev-parse --short=7 c-common) &&
+		common_new_oid=$(git rev-parse --short=7 HEAD) &&
+
+		git range-diff -s --abbrev=7 combined-old...combined-new >actual &&
+		cat >expect <<-EOF &&
+		1:  $old_only_oid < -:  ------- c-old-only
+		-:  ------- > 1:  $new_only_oid c-new-only
+		2:  $common_old_oid = 2:  $common_new_oid c-common
+		EOF
+		test_cmp expect actual &&
+
+		git range-diff -s --abbrev=7 --left-only combined-old...combined-new \
+			>actual &&
+		cat >expect <<-EOF &&
+		1:  $old_only_oid < -:  ------- c-old-only
+		2:  $common_old_oid = 2:  $common_new_oid c-common
+		EOF
+		test_cmp expect actual &&
+
+		git range-diff -s --abbrev=7 --right-only combined-old...combined-new \
+			>actual &&
+		cat >expect <<-EOF &&
+		-:  ------- > 1:  $new_only_oid c-new-only
+		2:  $common_old_oid = 2:  $common_new_oid c-common
+		EOF
+		test_cmp expect actual &&
+
+		git range-diff -s --abbrev=7 --matched-only combined-old...combined-new \
+			>actual &&
+		echo "2:  $common_old_oid = 2:  $common_new_oid c-common" >expect &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'ranges with pathspecs' '
 	git range-diff topic...mode-only-change -- other-file >actual &&
 	test_line_count = 2 actual &&