Merge branch 'ml/replace-auto-execok'

Merge branch 'ml/replace-auto-execok'

This addresses CVE-2025-46334, Git GUI malicious command injection on
Windows.

A malicious repository can ship versions of sh.exe or typical textconv
filter programs such as astextplain.  Due to the unfortunate design of
Tcl on Windows, the search path when looking for an executable always
includes the current directory.  The mentioned programs are invoked when
the user selects "Git Bash" or "Browse Files" from the menu.

* ml/replace-auto-execok:
  git-gui: override exec and open only on Windows
  git-gui: sanitize $PATH on all platforms
  git-gui: assure PATH has only absolute elements.
  git-gui: cleanup git-bash menu item
  git-gui: avoid auto_execok in do_windows_shortcut
  git-gui: avoid auto_execok for git-bash menu item
  git-gui: remove unused proc is_shellscript
  git-gui: remove special treatment of Windows from open_cmd_pipe
  git-gui: use only the configured shell
  git-gui: make _shellpath usable on startup
  git-gui: use [is_Windows], not bad _shellpath
  git-gui: _which, only add .exe suffix if not present

Signed-off-by: Johannes Sixt <j6t@kdbg.org>