mergetool: fallback to tool when guitool unavailable

In git-difftool, if the tool is called with --gui but `diff.guitool` is
not set, it falls back to `diff.tool`. Make git-mergetool also fallback
from `merge.guitool` to `merge.tool` if the former is undefined.

If git-difftool, when called with `--gui`, were to use
`get_configured_mergetool` in a future patch, it would also get the
fallback behavior in the following precedence:

1. diff.guitool
2. merge.guitool
3. diff.tool
4. merge.tool

The behavior for when difftool or mergetool are called without `--gui`
should be identical with or without this patch.

Note that the search loop could be written as

	sections="merge"
	keys="tool"
	if diff_mode
	then
		sections="diff $sections"
	fi
	if gui_mode
	then
		keys="guitool $keys"
	fi

	merge_tool=$(
		IFS=' '
		for key in $keys
		do
			for section in $sections
			do
				selected=$(git config $section.$key)
				if test -n "$selected"
				then
					echo "$selected"
					return
				fi
			done
		done)

which would make adding a mode in the future much easier. However,
adding a new mode will likely never happen as it is highly discouraged
so, as a result, it is written in its current form so that it is more
readable for future readers.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index dad607e186..5b61c10a9c 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -167,6 +167,25 @@ test_expect_success 'gui mergetool' '
 	git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'gui mergetool without merge.guitool set falls back to merge.tool' '
+	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)" = "master updated" &&
+	test "$(cat file2)" = "master new" &&
+	test "$(cat subdir/file3)" = "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