bug#41423: 27.0.91; eshell file completion in tramp dir is slow (3 minutes) [regression on pretest]

[Gregory Heytings]

> The root of this bug is that `eshell-complete-commands-list' loops 
> through all executables *on the remote server* with (while comps-in-path 
> ...). That means typically 1000-2000 commands to check, one by one, 
> hence the 3 minutes delay.
>
> The easy fix is to hit C-g, which stops this loop.

Another note: in fact this bug exists because 
`eshell-complete-commands-list' is, in this context, called in Emacs 27, 
but not in Emacs 26 and earlier.

The backtrace is:

* eshell-complete-commands-list()
  #f(compiled-function () #<bytecode 0x1e0009b1b5c5>)()
  pcomplete--here(#f(compiled-function () #<bytecode 0x1e0009b1b5c5>) nil nil 
nil)
  #f(compiled-function () #<bytecode 0x1fff46e348cd94>)()
  pcomplete-completions()
  pcomplete-completions-at-point()
  #f(compiled-function () #<bytecode 0xcbc6f2b2c706bdb>)()
  completion-in-region--postch()

In Emacs 26 `pcomplete--here' is called only once, with 
`pcomplete-entries' and `file-directory-p' in its `form' argument, and 
`pcomplete-completions' returns.

In Emacs 27 `pcomplete--here' is called twice with these arguments, and a 
third time with `eshell-complete-commands-list' as its `form' argument. 
It is this third call which takes about three minutes to complete.