bug#31749: 26.1; Improve eldoc message commands heuristic

[Carlos Pita <carlosjosepita@gmail.com>]

The current criterion essentially removes the echo area message after
any command not whitelisted in eldoc-message-commands.

This is problematic since the list of relevant commands is unknown in
advance for evert possible extension package. One could argue that
extension package maintainers are responsible then, but I have had many
problems with eldoc behaving suboptimally in conjunction with company
mode, elpy mode and others (which are all quality extensions) that I
wouldn't put the burden of this on the maintainers.

Now, I might be completely wrong, but why don't you just check if
(current-message) is nil or not? This way if any command has written to
the echo area eldoc won't override its message. You can still keep the
whitelist in order to force override by eldoc (although I don't see much
value in this). You have probably thought many times about this
alternative and rejected for reasons I'm unable to see, but it's worth
to intent ;)

---

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-05-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:	Manjaro Linux

Recent messages:
xy
eldoc-print-current-symbol-info
xy [5 times]
Auto-saving...
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completing-read-default: Command attempted to use minibuffer while in minibuffer
Quit [3 times]
xy [2 times]
Eldoc mode disabled in current buffer
xy [9 times]

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