Do not add spaces after completions ending in "-"

Some programs use this to separate things in a word, see
https://github.com/spf13/cobra/pull/1249#discussion_r563605048

Require the token to be at least length 2 for the no-space behavior,
for completions of "-" like for python.

CHANGELOG.rst

@@ -194,8 +194,7 @@ Interactive improvements
 -  The interactive reader now allows ending a line in a logical operators (``&&`` and ``||``) instead of complaining about a missing command
    (This was already syntactically valid, but interactive sessions didn't know about it yet).
 -  The prompt is reprinted after a background job exits (:issue:`1018`).
--  fish no longer inserts a space after a completion ending in ``.`` or
-   ``,`` is accepted (:issue:`6928`).
+-  fish no longer inserts a space after a completion ending in ``.``, ``,`` or ``-`` is accepted (:issue:`6928`).
 -  If a filename is invalid when first pressing Tab, but becomes valid, it will be completed properly on the next attempt (:issue:`6863`).
 - ``help string match/replace/<subcommand>`` will show the help for string subcommands (:issue:`6786`).
 -  ``fish_key_reader`` sets the exit status to 0 when used with ``--help`` or ``--version`` (:issue:`6964`).

src/complete.cpp

@@ -196,7 +196,7 @@ static complete_flags_t resolve_auto_space(const wcstring &comp, complete_flags_
     if (flags & COMPLETE_AUTO_SPACE) {
         new_flags &= ~COMPLETE_AUTO_SPACE;
         size_t len = comp.size();
-        if (len > 0 && (std::wcschr(L"/=@:.,", comp.at(len - 1)) != nullptr))
+        if (len > 1 && (std::wcschr(L"/=@:.,-", comp.at(len - 1)) != nullptr))
             new_flags |= COMPLETE_NO_SPACE;
     }
     return new_flags;

src/reader.cpp

@@ -1868,7 +1868,7 @@ static uint32_t get_best_rank(const completion_list_t &comp) {
 ///
 /// - If the list is empty, flash the terminal.
 /// - If the list contains one element, write the whole element, and if the element does not end on
-/// a '/', '@', ':', '.', ',' or a '=', also write a trailing space.
+/// a '/', '@', ':', '.', ',', '-' or a '=', also write a trailing space.
 /// - If the list contains multiple elements, insert their common prefix, if any and display
 /// the list in the pager.  Depending on terminal size and the length of the list, the pager
 /// may either show less than a screenfull and exit or use an interactive pager to allow the