Fix escaping bug in parse_command_arg()

The position wasn't being advanced properly when parsing unquoted
escapes. It seems to have gone unnoticed because most characters
are unescaped as themselves, but certain special characters aren't
(e.g. '-').

src/parse-command.c

@@ -172,7 +172,7 @@ char *parse_command_arg(const char *cmd, size_t len, bool tilde)
             if (pos == len) {
                 goto end;
             }
-            ch = cmd[pos];
+            ch = cmd[pos++];
             // Fallthrough
         default:
             string_add_byte(&buf, ch);