Before:
* hand write arg parse
* only accepts one suffix
After:
* use `arg_parse` to parse args
* accepts multi suffixes
Closes#9611.
(cherry picked from commit aa65856ee0)
__fish_complete_suffix accepts a first argument containing a
brace-expansion, like
__fish_complete_suffix '.{c,cpp,py}'
We're gonna be removing the `eval` that does that shortly, so let's
remove all uses in our code.
