t7610: add mergetool --gui tests

In 063f2bd (mergetool: accept -g/--[no-]gui as arguments,
2018-10-24), mergetool was taught the --gui option but no tests were
added to ensure that it was working properly. Add a test to ensure that
it works.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/t7610-mergetool.sh

@@ -145,6 +145,28 @@ test_expect_success 'custom mergetool' '
 	git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'gui mergetool' '
+	test_config merge.guitool myguitool &&
+	test_config mergetool.myguitool.cmd "(printf \"gui \" && cat \"\$REMOTE\") >\"\$MERGED\"" &&
+	test_config mergetool.myguitool.trustExitCode true &&
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	git submodule update -N &&
+	test_must_fail git merge master &&
+	( yes "" | git mergetool --gui both ) &&
+	( yes "" | git mergetool -g file1 file1 ) &&
+	( yes "" | git mergetool --gui file2 "spaced name" ) &&
+	( yes "" | git mergetool --gui subdir/file3 ) &&
+	( yes "d" | git mergetool --gui file11 ) &&
+	( yes "d" | git mergetool --gui file12 ) &&
+	( yes "l" | git mergetool --gui submod ) &&
+	test "$(cat file1)" = "gui master updated" &&
+	test "$(cat file2)" = "gui master new" &&
+	test "$(cat subdir/file3)" = "gui master new sub" &&
+	test "$(cat submod/bar)" = "branch1 submodule" &&
+	git commit -m "branch1 resolved with mergetool"
+'
+
 test_expect_success 'mergetool crlf' '
 	test_when_finished "git reset --hard" &&
 	# This test_config line must go after the above reset line so that