add a set --show flag

[krader1961]

Many of us have written something like my show function:

$ show y
not set: local $y
not set: global $y
not set: universal $y
$ set -l y argle bargle
$ show y
set: local unexported $y count=2
$y[1] length=5 value=|argle|
$y[2] length=6 value=|bargle|
not set: global $y
not set: universal $y

However, this has at least two drawbacks:

1. any local vars it sets shadows those in the surrounding context, and
2. you can't actually display the values of each scope since the $var syntax provides no way to ask for the value(s) from a specific scope.

These drawbacks means it is much harder to give simple commands to help someone understand why their set commands aren't having the desired outcome. Such as in issue #4263. So I propose adding -S/--show flags to set to do the same thing as my show function but without the drawbacks. This would also be extremely useful in our unit tests where we also have a show function and employ other techniques to determine if a var is (un)set correctly.

[krader1961]

I am strongly inclined to include the fix for this in fish 2.7.0 despite the fact I have recently argued that other changes be deferred to 3.0.0. In this case the implementation is borderline trivial and no existing behavior is altered. If the user doesn't use the new --show flag then they can't be affected by the change I'd like to merge and will send out for review in a few minutes. And this feature is likely to be extremely useful in supporting people still using fish 2.x (assuming we can get them to upgrade to 2.7) and has zero risk of breaking anything. If you disagree please speak up.

[krader1961]

Sigh! I'm not going to merge this into master unless there is a deafening cry to do so. That's because I've already made some other cleanups to src/builtin_set.cpp that the change is based on that are not present in the master branch. Which means that merging this into the master branch, then merging that into major, is going to be a PITA. My attempt to git cherry-pick 0e8af72ca23a0f6912c05865730d0ee0875209ac resulted in an impenetrable merge conflict.

[krader1961]

Note that I did ultimately backport this enhancement into the fish 2.7 development (aka master) branch. Since doing so does not alter any existing behavior and is useful for debugging problems.