BUG #15002: Unexpected behaviour in psql \r command

The following bug has been logged on the website:

Bug reference:      15002
Logged by:          Petr Korobeinikov
Email address:      pkorobeinikov@gmail.com
PostgreSQL version: 10.0
Operating system:   Any (reproduced in macOS Sierra, Slackware)
Description:

Dear hackers!

Unexpected behaviour of \r subcommand has been introduced in 10.0
(e984ef5861df4bc9733b36271d05763e82de7c04).

Previously (at least in 9.6) previous_buf hasn’t been used for any kinds of
\e subcommand.
Now if query_buf is empty previous buffer contents is used.

It makes useless \r subcommand in cases when clean buffer required in next
\e.
Multiple calls of \r takes no effect.

Code snippet:
```
794 +
795 +               /* Applies to previous query if current buffer is empty
*/
796 +               copy_previous_query(query_buf, previous_buf);
797 +
798                 if (do_edit(fname, query_buf, lineno, NULL))
799                     status = PSQL_CMD_NEWEDIT;
```