bug#48356: 28.0.50; choose-completion discards the suffix after the completion boundary

[Daniel Mendler]

When selecting a candidate the suffix after the completion boundary is
discarded by `choose-completion`/`choose-completion-string`.
`choose-completion` is invoked when a candidate in the *Completions*
buffer is selected with the mouse or RET.

For example when completing a file path "~/emacs/master/li|/calc", where
"|" is the cursor, and then the candidate "lisp" is selected in the
*Completions* buffer, the result is "~/emacs/master/lisp/". The prefix
"~/emacs/master/" is prepended to the selected candidate, but the suffix
"/calc" is discarded.

`choose-completion-string` contains logic which checks if the resulting
string equals the car of the completion boundary. In that case the
minibuffer is not exited.

I propose the following change to the existing logic: When
selecting a candidate with `choose-completion` and a suffix is present,
the minibuffer should not be exited (completion continues) and the
suffix is preserved.