Merge branch 'js/gitk-git-gui-harden-exec-open' into maint-2.43

This merges in fixes for CVE-2025-27614, CVE-2025-27613, CVE-2025-46334,
and CVE-2025-46835 targeting Gitk and Git GUI.

* js/gitk-git-gui-harden-exec-open: (41 commits)
  git-gui: sanitize 'exec' arguments: convert new 'cygpath' calls
  git-gui: do not mistake command arguments as redirection operators
  git-gui: introduce function git_redir for git calls with redirections
  git-gui: pass redirections as separate argument to git_read
  git-gui: pass redirections as separate argument to _open_stdout_stderr
  git-gui: convert git_read*, git_write to be non-variadic
  git-gui: override exec and open only on Windows
  gitk: sanitize 'open' arguments: revisit recently updated 'open' calls
  git-gui: use git_read in githook_read
  git-gui: sanitize $PATH on all platforms
  git-gui: break out a separate function git_read_nice
  git-gui: assure PATH has only absolute elements.
  git-gui: remove option --stderr from git_read
  git-gui: cleanup git-bash menu item
  git-gui: sanitize 'exec' arguments: background
  git-gui: avoid auto_execok in do_windows_shortcut
  git-gui: sanitize 'exec' arguments: simple cases
  git-gui: avoid auto_execok for git-bash menu item
  git-gui: treat file names beginning with "|" as relative paths
  git-gui: remove unused proc is_shellscript
  git-gui: remove git config --list handling for git < 1.5.3
  git-gui: remove special treatment of Windows from open_cmd_pipe
  git-gui: remove HEAD detachment implementation for git < 1.5.3
  git-gui: use only the configured shell
  git-gui: remove Tcl 8.4 workaround on 2>@1 redirection
  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
  gitk: encode arguments correctly with "open"
  gitk: sanitize 'open' arguments: command pipeline
  gitk: collect construction of blameargs into a single conditional
  gitk: sanitize 'open' arguments: simple commands, readable and writable
  gitk: sanitize 'open' arguments: simple commands with redirections
  gitk: sanitize 'open' arguments: simple commands
  gitk: sanitize 'exec' arguments: redirect to process
  gitk: sanitize 'exec' arguments: redirections and background
  gitk: sanitize 'exec' arguments: redirections
  gitk: sanitize 'exec' arguments: 'eval exec'
  gitk: sanitize 'exec' arguments: simple cases
  gitk: have callers of diffcmd supply pipe symbol when necessary
  gitk: treat file names beginning with "|" as relative paths
  ...

Signed-off-by: Taylor Blau <me@ttaylorr.com>