Fix Fish shell assignments on array env variables

[hugomg]

• Correctly handle the case of the variable being unset
  • This fixes the "missing manpages" bug.
• Do not put quotes around array expansions:
  set -gx PATH $PATH "/blah" # Good
  set -gx PATH "$PATH" "/blah" # Bad
• Add logic for CDPATH, for future-proofing
  • It is the only other special array variable we have
    to potentially worry about.

[hugomg]

Hmm, just noticed that there are also assignments on "/src/client/opamConfigCommand.ml". Dunno what would be the best way to proceed without duplicating a lot of code.

[hugomg]

I couldn't fully understand why there is separate logic for generating the shell scripts that go on variables.fish and on opam config env. Right now, the code that generates variables.fish can handle assignments with variable expansions on the RHS (things like PATH="$PATH:/blah") and the code for opam config env does NOT and assumes that the directory names in the path lists are specified in full.

If this difference between the two functions is not a problem then I think this set of patches should fix #2165

[AltGr]

Great, thanks!
opam config env is different because it can afford to be a bit more clever: it expands $PATH, detects previous instances of opam paths tied to the same opam root and replaces them. And re-sets the variable in full.
variables.fish is much simpler, and is normally loaded only once at shell init, so expanding directly over $PATH using shell expansion is fine. The fish issue had already been (mostly?) fixed for opam config env, but not the variables.fish variant, it seems.

[avsm]

This also seems like a 1.2.3 point fix candidate?

[johshoff]

I'd like to support @avsm's suggestion to make a point release. This breaks the configuration of fish shells when installing the latest release, 1.2.2 (released a year ago, so I'm assuming this is what most package managers have).

[cartazio]

when is this being merged? @avsm ?
seems like its still biting folks

[avsm]

It's included in the forthcoming opam 2.0 release, which has just been tagged as rc1