Don't chomp foo= when completing foo=br

[cstyles]

Description

complete_param_expand knows how to handle cases like foo=br so we
don't need to bother sending just the br part. Furthermore, sending
just br is incorrect because we will end up replacing the entirety of
foo=br with the result of the completion. That is, foo=br will be
replaced with bar instead of being completed to foo=bar.

Fixes a bug where the following

$ foo=/vr<TAB>

will be completed to just

$ /var/

instead of

$ foo=/var/

This doesn't happen when the command is being invoked with env like env foo=/vr<TAB>. It also only occurs when the completion requires replacement. So foo=/va will correctly be completed to foo=/var/.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

Right, a fuzzy completion will set COMPLETE_REPLACES_TOKEN, so the entire token is replaced.
We already special-case separators like = in the parameter completion, so we can just reuse that. Thanks!

[cstyles]

No problem!

If it's not too much to ask, could you label this PR as hacktoberfest-accepted? I'm trying to get a free T-shirt. :)