bug#66394: 29.1; Make register-read-with-preview more useful

[Bastien]

I use registers ~100 times a day, so enhancements here are very
welcome, thanks!

Thierry Volpiatto <thievol@posteo.net> writes:

> A minibuffer is used now instead of read-key.

I wonder about this, though. It badly hinders my usual flow, where I
do remember what registers I use and like to store new ones quickly.

When a register is empty, I believe it's more efficient to just read
the key and store the content in the register directly.

E.g. if the "a" contains "A string" and "b" is an empty register:

- C-x r s would display the preview and copy the region to the "b"
  register as soon as the "b" key is hit (using read-key).

- C-x r s would display the preview and if the user hits "a", it will
  warn about overwriting the existing register and RET can confirm.

This supposes using read-key by default and switch to using a
minibuffer when the user hits keys for existing registers.

What do you think?

-- 
 Bastien