Add support for fish shell

[WaffleLapkin]

This is an attempt to fix#478.

I'm not sure how to test if it works though, do I have to install local build of rustup in a vm somehow? 🤔

[rbtcollins]

Sorry for not getting to this earlier.

We generally try to write to all the places that a shell might read. The helper we wrote is idempotent to support this. What would happen if that was done here?

[WaffleLapkin]

Hm, to be honest I've lost all the context since I've opened this PR... Do you mean I should just call rcfiles here?

[rbtcollins]

approximately - look at zsh for the typical pattern

[WaffleLapkin]

@rbtcollins the current version calls rcfiles here.

Is there anything more that I can do here?

[WaffleLapkin]

Hm, I've also noticed that the default env script doesn't work with fish, so I've written a custom one. . ~/path/to/repo/rustup/src/cli/self_update/env.fish does add the desired path to the $PATH (after the {cargo_bin} replacement of course), so I think this should work.

[rami3l]

The new version looks good!

@rbtcollins may I ask why this has been blocked from merging for so long?

[srcrip]

Yeah same, is there any reason this can't be merged?

[djc]

@sevensidedmarble there's no point in piling on -- please bear with us while we try to increase review capacity.

[YoutacRandS-VA]

Loving the changes/updates