`__fish_complete_path` filtering ignores prefixes like `--anything=`

https://github.com/fish-shell/fish-shell/issues/12885 should get reopened but I do not have permissions.

The issue still persists under fish 4.9.0:

~/r/c/bug (master)> echo $version
4.9.0
 ~/r/c/bug (master)> ls -1
--bar
foo
 ~/r/c/bug (master)> __fish_complete_path --bar
--bar
 ~/r/c/bug (master)> __fish_complete_path --bar=
--bar=foo
--bar=--bar

Expected behavior: __fish_complete_path --bar= returns nothing since there are no file names matching the prefix --bar=.