bug#66137: 29.1.50; pcomplete/git duplicates branch name prefix

[Daniel Mendler]

Recipe:

1. Open "M-x shell" inside the "emacs" git repository.
2. Enter "git log origin/ TAB".
3. *Completions* window is shown, with candidates like
   "origin/emacs-29".
4. Switch to *Completions* window with "C-x o".
5. Select a candidate with RET on "origin/emacs-29". The shell prompt is
   "git log origin/origin/emacs-29".

The problem seems to be either that the candidates are unnecessarily
prefixed with "origin/" or that the completion boundaries are not
set for the completion table. This would ensure that "origin/" is
replaced properly with "origin/emacs-29" after selection.

For comparison when I use C-x C-f in the ~/emacs/ directory and press
TAB, the candidates displayed in the *Completions* buffer (e.g. "lisp/")
don't include the "~/emacs/" prefix, such that selecting one of the
sub directories results in the correct path "~/emacs/lisp/".

In GNU Emacs 29.1.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw scroll bars) of 2023-09-19 built on pyramis
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)