difftool: add a missing space to the run_dir_diff() comments

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/difftool.c

@@ -558,7 +558,7 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	}
 
 	/*
-	 * Symbolic links require special treatment.The standard "git diff"
+	 * Symbolic links require special treatment. The standard "git diff"
 	 * shows only the link itself, not the contents of the link target.
 	 * This loop replicates that behavior.
 	 */