string match with -r (regex) and -v (invert) does not respect -n (index).

Reproduction Steps:

$ string match -r -v -n a b

Expected behavior:

fish should report the start position and length when -n is given, as it does without -v.

Observed behavior:

$ string match -r -v -n a b
b

Additional information:

I think this was introduced with the addition of the (-v | --invert) options to string. The problem appears to be here: https://github.com/fish-shell/fish-shell/blob/master/src/builtin_string.cpp#L378-L381 where the argument is reported without checking for opts.index as is done below.

Fish version: fish, version 2.3.0

Operating system: Arch linux, fish installed from the official package

For 2.3.1 release:

• Update release notes