fish: support -o short option correctly

Closes #7254.

CHANGELOG.rst

@@ -40,6 +40,8 @@ Notable improvements and fixes
     dmesg -w | string replace foo bar
 
 - A number of new debugging categories have been added, including ``config``, ``path``, ``reader`` and ``screen`` (#6511). See the output of ``fish --print-debug-categories`` for the full list.
+- The ``-o`` short option to fish, for ``--debug-output``, works correctly instead of producing an
+  invalid option error (#7254).
 - ``set`` and backgrounded jobs no longer overwrite ``$pipestatus``.
 
 Syntax changes and new commands

src/fish.cpp

@@ -259,7 +259,7 @@ int run_command_list(parser_t &parser, std::vector<std::string> *cmds, const io_
 
 /// Parse the argument list, return the index of the first non-flag arguments.
 static int fish_parse_opt(int argc, char **argv, fish_cmd_opts_t *opts) {
-    static const char *const short_opts = "+hPilnvc:C:p:d:f:D:";
+    static const char *const short_opts = "+hPilnvc:C:p:d:f:D:o:";
     static const struct option long_opts[] = {
         {"command", required_argument, nullptr, 'c'},
         {"init-command", required_argument, nullptr, 'C'},