Added argparse support for arguments with multiple optional values.

This commit fixes #8432 by adding put =* in an option spec to indicate that the
option takes an optional value, where subsequent uses of the option accumulate
the value (so the parsing behaviour is like =?, but the _flag_ variables are
appended to like =+). If the option didn't have a value, it appends an empty
string. As an example,. long=* -- --long=1 --long will execute
set -l _flag_long 1 '' (i.e. count $_flag_long is 2), whereas with =? instead,
you'd get set -l _flag_long (i.e. count $_flag_long is 0).

As a use case, I'm aware of git clone which has a
--recurse-submodules=[<pathspec>]: if you use it without a value, it operates on
all submodules, with a value, it operates on the given submodule.

The fish_opt function will generate an =* option spec when given both the
--optional-val and --multiple-vals options (previously, doing so was an error).
fish_opt now also accepts -m as an abbreviation for --multiple-vals, to go with
the pre-existing -o and -r abbreviations for --optional-val and --required-val.

doc_src/cmds/argparse.rst

@@ -69,7 +69,7 @@ The following ``argparse`` options are available. They must appear before all *O
     This option implies **--move-unknown**, unless **--ignore-unknown** is also given.
     This will modify the parsing behaviour of unknown options depending on the value of *KIND*:
 
-        - **optional** (the default), allows each unknown option to take an optional argument (i.e. as if it had ``=?`` in its option specification). For example, ``argparse --ignore-unknown --unknown-arguments=optional ab -- -u -a -ub`` will set ``_flag_a`` but *not* ``_flag_b``, as the ``b`` is treated as an argument to the second use of ``-u``.
+        - **optional** (the default), allows each unknown option to take an optional argument (i.e. as if it had ``=?`` or ``=*`` in its option specification). For example, ``argparse --ignore-unknown --unknown-arguments=optional ab -- -u -a -ub`` will set ``_flag_a`` but *not* ``_flag_b``, as the ``b`` is treated as an argument to the second use of ``-u``.
 
         - **required** requires each unknown option to take an argument (i.e. as if it had ``=`` or ``=+`` in its option specification). If the above example was changed to use ``--unknown-arguments=required``, *neither* ``_flag_a`` nor ``_flag_b`` would be set: the ``-a`` will be treated as an argument to the first use of ``-u``, and the ``b`` as an argument to the second.
 
@@ -146,7 +146,9 @@ Each option specification consists of:
 
     - **=?** if it takes an optional value and only the last instance of the flag is saved, or
 
-    - **=+** if it requires a value and each instance of the flag is saved.
+    - **=+** if it requires a value and each instance of the flag is saved, or
+
+    - **=\*** if it takes an optional value *and* each instance of the flag is saved, storing the empty string when the flag was not given a value.
 
 - Optionally a ``&``, indicating that the option and any attached values are not to be saved in ``$argv`` or ``$argv_opts``. This does not affect the the ``_flag_`` variables.
 
@@ -170,7 +172,7 @@ This does not read numbers given as ``+NNN``, only those that look like flags -
 Note: Optional arguments
 ------------------------
 
-An option defined with ``=?`` can take optional arguments. Optional arguments have to be *directly attached* to the option they belong to.
+An option defined with ``=?`` or ``=*`` can take optional arguments. Optional arguments have to be *directly attached* to the option they belong to.
 
 That means the argument will only be used for the option if you use it like::
 
@@ -245,6 +247,8 @@ Some *OPTION_SPEC* examples:
 
 - ``n/name=?`` means that both ``-n`` and ``--name`` are valid. It accepts an optional value and can be used at most once. If the flag is seen then ``_flag_n`` and ``_flag_name`` will be set with the value associated with the flag if one was provided else it will be set with no values.
 
+- ``n/name=*`` is similar, but the flag can be used more than once. If the flag is seen then ``_flag_n`` and ``_flag_name`` will be set with the values associated with each occurence. Each value will be the value given to the option, or the empty string if no value was given.
+
 - ``name=+`` means that only ``--name`` is valid. It requires a value and can be used more than once. If the flag is seen then ``_flag_name`` will be set with the values associated with each occurrence.
 
 - ``x`` means that only ``-x`` is valid. It is a boolean that can be used more than once. If it is seen then ``_flag_x`` will be set as above.