Minor: env/exec completion includes fish functions

[ilyagr]

I'm using fish 3.6.1 (from the SUSE repo as recommended in fishshell.com) on Debian testing. It happens even if I run fish with sh -c 'env HOME=$(mktemp -d) fish'.

If I enter env funce<tab>, it completes to env funced. However, env funced does not work since funced is a fish function, not an external command.

The same thing happens with exec.

I think that both env and exec should ideally only complete actual external commands, not fish functions.

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It is possible that some people are relying on this bug. For example, something like the following would work better with this bug present than without it:

function k --wraps=exec
  EDITOR=kak $argv
end

As @zanchey pointed out on Gitter, this usecase would probably be best served by abbreviations.