git-mergetool--lib.sh: use TOOL_MODE when erroring about unknown tool

In git-mergetool--lib.sh::get_merge_tool_path, we check if the chosen
tool is valid via valid_tool and exit with an error message if not. This
error message mentions "Unknown merge tool", even if the command the
user tried was 'git difftool --tool=unknown'. Use the global 'TOOL_MODE'
variable for a more correct error message.

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

git-mergetool--lib.sh

@@ -474,7 +474,7 @@ get_merge_tool_path () {
 	merge_tool="$1"
 	if ! valid_tool "$merge_tool"
 	then
-		echo >&2 "Unknown merge tool $merge_tool"
+		echo >&2 "Unknown $TOOL_MODE tool $merge_tool"
 		exit 1
 	fi
 	if diff_mode