[gdb/cli] Don't prefill for operate-and-get-next of last command

Consider operate-and-get-next [1] in bash:
...
$ <echo 1>echo 1<enter>
1
$ <echo 2>echo 2<enter>
2
$ <Ctrl-r>(reverse-i-search)`': <echo 1>echo 1<Ctrl-o>
1
$ echo 2<Ctrl-o>
2
$ echo 1
...

So, typing Ctrl-o:
- executes the recalled command, and
- prefills the next one (which then can be executed again with Ctrl-o).

We have the same functionality in gdb, but when recalling the last command
from history with bash we have no prefill:
...
$ <echo 1>echo 1<enter>
1
$ <Ctrl-r>(reverse-i-search)`': <echo 1>echo 1<Ctrl-o>
1
$
...
but with gdb do we have a prefill:
...
(gdb) echo 1\n
1
(gdb) <Ctrl-r>(reverse-i-search)`': <echo 1>echo 1\n<Ctrl-o>
1
(gdb) echo 1\n
...

Following the principle of least surprise [2], I think gdb should do what bash
does.

Fix this by:
- signalling this case in gdb_rl_operate_and_get_next using
  "operate_saved_history = -1", and
- handling operate_saved_history == -1 in
  gdb_rl_operate_and_get_next_completion.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR cli/32485
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32485

[1] https://www.man7.org/linux/man-pages/man3/readline.3.html
[2] https://en.wikipedia.org/wiki/Principle_of_least_astonishment

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