bug#67688: 30.0.50; ffap prepends buffers default-directory to guessed file name in case of tramp's file name syntax

[Gregor Zattler]

Dear Emacs developers, following this recipe (in
some Gnu/Linux shell):

$ cd /tmp
tmp$ emacs -Q -nw
# type /ssh:user@server:/home/test
# type M-x ffap RET

the proposed file name in minibuffer then is:
/tmp/ssh:user@server:/home/test
instead of
/ssh:user@server:/home/test

Therefore it's not possible to just hit RET in order
to open that file.  But that would be nice.

Thanks for your attention, Gregor

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu,
 cairo version 1.16.0) of 2023-12-02 built on no
Repository revision: fb4b0b30a24e8192e71c1425dc8a295cf39f64ff
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure --infodir=/usr/share/info/emacs --with-json
 --with-file-notification=yes --with-libsystemd
 --with-cairo --with-x=yes --with-x-toolkit=no
 --without-toolkit-scroll-bars --without-gsettings
 --enable-checking=yes,glyphs
 --enable-check-lisp-object-type
 --with-native-compilation 'CFLAGS=-g3 -O3
 
-ffile-prefix-map=/home/grfz/src/emacs-master--fb4b0b30a24e8192e71c1425dc8a295cf39f64ff--2023-12-02T22-47+01-00=.
 -fstack-protector-strong
 -Wformat -Werror=format-security ''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM
HARFBUZZ JPEG JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD
LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY INOTIFY
OLDXMENU PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS
TIFF WEBP X11 XDBE XIM XINPUT2 XPM ZLIB

Important settings:
  value of $LC_ALL:
  value of $LC_COLLATE: de_DE.utf8
  value of $LC_CTYPE: de_DE.utf8
  value of $LC_MESSAGES: POSIX
  value of $LC_MONETARY: de_DE.utf8
  value of $LC_NUMERIC: de_DE.utf8
  value of $LC_TIME: de_DE.utf8
  value of $LANG: de_DE.utf8
  locale-coding-system: utf-8-unix