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>

Documentation/git-mergetool.txt

@@ -83,7 +83,9 @@ success of the resolution after the custom tool has exited.
 --gui::
 	When 'git-mergetool' is invoked with the `-g` or `--gui` option
 	the default merge tool will be read from the configured
-	`merge.guitool` variable instead of `merge.tool`.
+	`merge.guitool` variable instead of `merge.tool`. If
+	`merge.guitool` is not set, we will fallback to the tool
+	configured under `merge.tool`.
 
 --no-gui::
 	This overrides a previous `-g` or `--gui` setting and reads the

git-mergetool--lib.sh

@@ -354,19 +354,36 @@ guess_merge_tool () {
 }
 
 get_configured_merge_tool () {
-	if gui_mode
-	then
-		gui_prefix=gui
-	fi
-
-	# Diff mode first tries diff.(gui)tool and falls back to merge.(gui)tool.
-	# Merge mode only checks merge.(gui)tool
+	keys=
 	if diff_mode
 	then
-		merge_tool=$(git config diff.${gui_prefix}tool || git config merge.${gui_prefix}tool)
+		if gui_mode
+		then
+			keys="diff.guitool merge.guitool diff.tool merge.tool"
+		else
+			keys="diff.tool merge.tool"
+		fi
 	else
-		merge_tool=$(git config merge.${gui_prefix}tool)
+		if gui_mode
+		then
+			keys="merge.guitool merge.tool"
+		else
+			keys="merge.tool"
+		fi
 	fi
+
+	merge_tool=$(
+		IFS=' '
+		for key in $keys
+		do
+			selected=$(git config $key)
+			if test -n "$selected"
+			then
+				echo "$selected"
+				return
+			fi
+		done)
+
 	if test -n "$merge_tool" && ! valid_tool "$merge_tool"
 	then
 		echo >&2 "git config option $TOOL_MODE.${gui_prefix}tool set to unknown tool: $merge_tool"

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