merge: refuse --commit with --squash

Convert option_commit to tristate, representing the states of
'default/untouched', 'enabled-by-cli', 'disabled-by-cli'. With this in
place, check whether option_commit was enabled by cli when squashing a
merge. If so, error out, as this is not supported.

Previously, when --squash was supplied, 'option_commit' was silently
dropped. This could have been surprising to a user who tried to override
the no-commit behavior of squash using --commit explicitly.

Add a note to the --squash option for git-merge to clarify the
incompatibility, and add a test case to t7600-merge.sh

Cc: Junio C Hamano <gitster@pobox.com>
Cc: Rafael Ascensão <rafa.almas@gmail.com>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Vishal Verma <vishal@stellar.sh>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -90,6 +90,8 @@ merge.
 +
 With --no-squash perform the merge and commit the result. This
 option can be used to override --squash.
++
+With --squash, --commit is not allowed, and will fail.
 
 -s <strategy>::
 --strategy=<strategy>::

builtin/merge.c

@@ -57,7 +57,7 @@ static const char * const builtin_merge_usage[] = {
 };
 
 static int show_diffstat = 1, shortlog_len = -1, squash;
-static int option_commit = 1;
+static int option_commit = -1;
 static int option_edit = -1;
 static int allow_trivial = 1, have_message, verify_signatures;
 static int overwrite_ignore = 1;
@@ -1304,9 +1304,19 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (squash) {
 		if (fast_forward == FF_NO)
 			die(_("You cannot combine --squash with --no-ff."));
+		if (option_commit > 0)
+			die(_("You cannot combine --squash with --commit."));
+		/*
+		 * squash can now silently disable option_commit - this is not
+		 * a problem as it is only overriding the default, not a user
+		 * supplied option.
+		 */
 		option_commit = 0;
 	}
 
+	if (option_commit < 0)
+		option_commit = 1;
+
 	if (!argc) {
 		if (default_to_upstream)
 			argc = setup_with_upstream(&argv);

t/t7600-merge.sh

@@ -525,6 +525,12 @@ test_expect_success 'combining --squash and --no-ff is refused' '
 	test_must_fail git merge --no-ff --squash c1
 '
 
+test_expect_success 'combining --squash and --commit is refused' '
+	git reset --hard c0 &&
+	test_must_fail git merge --squash --commit c1 &&
+	test_must_fail git merge --commit --squash c1
+'
+
 test_expect_success 'option --ff-only overwrites --no-ff' '
 	git merge --no-ff --ff-only c1 &&
 	test_must_fail git merge --no-ff --ff-only c2