mergetool: accept -g/--[no-]gui as arguments

In line with how difftool accepts a -g/--[no-]gui option, make mergetool
accept the same option in order to use the `merge.guitool` variable to
find the default mergetool instead of `merge.tool`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Anmol Mago <anmolmago@gmail.com>
Signed-off-by: Brian Ho <briankyho@gmail.com>
Signed-off-by: David Lu <david.lu97@outlook.com>
Signed-off-by: Ryan Wang <shirui.wang@hotmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Denton-L

Documentation/git-mergetool.txt

@@ -79,6 +79,17 @@ success of the resolution after the custom tool has exited.
 	Prompt before each invocation of the merge resolution program
 	to give the user a chance to skip the path.
 
+-g::
+--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`.
+
+--no-gui::
+	This overrides a previous `-g` or `--gui` setting and reads the
+	default merge tool will be read from the configured `merge.tool`
+	variable.
+
 -O<orderfile>::
 	Process files in the order specified in the
 	<orderfile>, which has one shell glob pattern per line.

git-mergetool--lib.sh

@@ -350,17 +350,23 @@ guess_merge_tool () {
 }
 
 get_configured_merge_tool () {
-	# Diff mode first tries diff.tool and falls back to merge.tool.
-	# Merge mode only checks merge.tool
+	# If first argument is true, find the guitool instead
+	if test "$1" = true
+	then
+		gui_prefix=gui
+	fi
+
+	# Diff mode first tries diff.(gui)tool and falls back to merge.(gui)tool.
+	# Merge mode only checks merge.(gui)tool
 	if diff_mode
 	then
-		merge_tool=$(git config diff.tool || git config merge.tool)
+		merge_tool=$(git config diff.${gui_prefix}tool || git config merge.${gui_prefix}tool)
 	else
-		merge_tool=$(git config merge.tool)
+		merge_tool=$(git config merge.${gui_prefix}tool)
 	fi
 	if test -n "$merge_tool" && ! valid_tool "$merge_tool"
 	then
-		echo >&2 "git config option $TOOL_MODE.tool set to unknown tool: $merge_tool"
+		echo >&2 "git config option $TOOL_MODE.${gui_prefix}tool set to unknown tool: $merge_tool"
 		echo >&2 "Resetting to default..."
 		return 1
 	fi

git-mergetool.sh

@@ -9,7 +9,7 @@
 # at the discretion of Junio C Hamano.
 #
 
-USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [-O<orderfile>] [file to merge] ...'
+USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [-g|--gui|--no-gui] [-O<orderfile>] [file to merge] ...'
 SUBDIRECTORY_OK=Yes
 NONGIT_OK=Yes
 OPTIONS_SPEC=
@@ -389,6 +389,7 @@ print_noop_and_exit () {
 
 main () {
 	prompt=$(git config --bool mergetool.prompt)
+	gui_tool=false
 	guessed_merge_tool=false
 	orderfile=
 
@@ -414,6 +415,12 @@ main () {
 				shift ;;
 			esac
 			;;
+		--no-gui)
+			gui_tool=false
+			;;
+		-g|--gui)
+			gui_tool=true
+			;;
 		-y|--no-prompt)
 			prompt=false
 			;;
@@ -443,7 +450,7 @@ main () {
 	if test -z "$merge_tool"
 	then
 		# Check if a merge tool has been configured
-		merge_tool=$(get_configured_merge_tool)
+		merge_tool=$(get_configured_merge_tool $gui_tool)
 		# Try to guess an appropriate merge tool if no tool has been set.
 		if test -z "$merge_tool"
 		then