Mention env in error for bash-style environment overrides

[dag]

The question of how to set environment exports for a single command line came up twice today in the IRC channel, and in multiple issues submitted here lately. It's a common gotcha.

Currently if you try the bash syntax, you get a nice error except it's not too relevant:

> EDITOR=vim git commit
fish: Unknown command “EDITOR=vim”. Did you mean “set EDITOR vim”? For
information on assigning values to variables, see the help section on the set
command by typing “help set”.

Perhaps fish could detect if there was any tokens following the environment assignment (ie. a command) and in that case instead print an error message suggesting env EDITOR=vim git commit. If this is hard to get right, perhaps the error above could simply be changed to mention both possibilities.

Cf. #587, #438 etc.

[ridiculousfish]

Good idea.

[zanchey]

I've also added an FAQ as it is genuinely frequently asked!

zanchey/fish-shell@ce12da1f6c

Part of pull request #781

[jdevera]

Neither of the proposed solutions work when the command being called is a function. I'll log it.

[ridiculousfish]

2f33e59 improves the error message for this scenario. Thanks for the idea zanchey!