Merge branch 'dl/difftool-mergetool'

Update "git difftool" and "git mergetool" so that the combinations
of {diff,merge}.{tool,guitool} configuration variables serve as
fallback settings of each other in a sensible order.

* dl/difftool-mergetool:
  difftool: fallback on merge.guitool
  difftool: make --gui, --tool and --extcmd mutually exclusive
  mergetool: fallback to tool when guitool unavailable
  mergetool--lib: create gui_mode function
  mergetool: use get_merge_tool function
  t7610: add mergetool --gui tests
  t7610: unsuppress output

git-mergetool--lib.sh

@@ -80,14 +80,18 @@ show_tool_names () {
 	}
 }
 
-diff_mode() {
+diff_mode () {
 	test "$TOOL_MODE" = diff
 }
 
-merge_mode() {
+merge_mode () {
 	test "$TOOL_MODE" = merge
 }
 
+gui_mode () {
+	test "$GIT_MERGETOOL_GUI" = true
+}
+
 translate_merge_tool_path () {
 	echo "$1"
 }
@@ -351,20 +355,36 @@ guess_merge_tool () {
 }
 
 get_configured_merge_tool () {
-	# If first argument is true, find the guitool instead
-	if test "$1" = true
-	then
-		gui_prefix=gui
-	fi
-
-	# Diff mode first tries diff.(gui)tool and falls back to merge.(gui)tool.
-	# Merge mode only checks merge.(gui)tool
+	keys=
 	if diff_mode
 	then
-		merge_tool=$(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"
@@ -404,14 +424,17 @@ get_merge_tool_path () {
 }
 
 get_merge_tool () {
+	is_guessed=false
 	# Check if a merge tool has been configured
 	merge_tool=$(get_configured_merge_tool)
 	# Try to guess an appropriate merge tool if no tool has been set.
 	if test -z "$merge_tool"
 	then
 		merge_tool=$(guess_merge_tool) || exit
+		is_guessed=true
 	fi
 	echo "$merge_tool"
+	test "$is_guessed" = false
 }
 
 mergetool_find_win32_cmd () {