bug#63829: 29.0.90; project-find-file's future history breaks with common-parent-directory

[Spencer Baugh <sbaugh@janestreet.com>]

1. emacs -Q
2. Open a project where project-files only returns files in a certain
subdirectory.  For example, a git repo /repo where the only file is
"dir/file.txt".
3. Open dir/file.txt
4. C-x p f   ;; project-find file
5. Observe that the prompt is "Find file in /repo/dir: " which correctly
contains the common parent directory between all the paths returned by
project-files.
6. M-n       ;; next-history-element
7. The minibuffer now contains "dir/file.txt".  RET will fail to
open the file.

Instead, the common parent directory should be stripped from the "future
history" element.

(As a separate point: I ran into this while adding a feature for
switching between projects with similar directory structures.  I want to
include the relative path in the starting project in the "future
history", so that when you have a file in projectA open, you can switch
to the same file in projectB with C-x p p f M-n RET.  For example,
switching between the same file in multiple clones of Emacs.  But sadly
the future history doesn't work properly right now even in a single
project)


In GNU Emacs 29.0.90 (build 3, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2023-05-17 built on
 igm-qws-u22796a
Repository revision: 4d08492296c2a6d2910f2b740c2d2508275458fc
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: CentOS Linux 7 (Core)

Configured using:
 'configure --with-x-toolkit=lucid --with-gif=ifavailable'

Configured features:
CAIRO DBUS FREETYPE GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND
SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XINPUT2 XPM LUCID
ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix