[PATCH v3 1/1] git-gui: handle missing worktree and separated gitdir

When git-gui is started from a directory that Git recognizes as a valid repository but the working tree is not accessible (e.g., a separated gitdir created by `git clone --separate-git-dir`, a bare repository, or a case where the worktree directory was removed), it previously called `rev-parse --show-toplevel` without error handling, causing a fatal Tcl error ("this operation must be run in a work tree").

Wrap the call in a `catch` and handle the failure as follows:

- For bare repositories, keep `_gitworktree` empty so that the
  existing `is_bare` check shows "Cannot use bare repository" and
  exits.  No behavioral change.

- For non‑bare repositories, try to locate the worktree from the
  parent directory using `git -C $parent rev-parse --show-toplevel`.
  If the parent is a valid worktree, change to it; this covers the
  legitimate case of starting git-gui from within the .git
  subdirectory of a normal working tree.

- If the parent directory is not a worktree, refuse to start with
  a clear error message.  This prevents dangerous operations in a
  separated gitdir, where ordinary Git commands like `git status`
  would themselves refuse to run.

The approach intentionally avoids two pitfalls:

  - Testing `--is-inside-git-dir` before calling `--show-toplevel`
    would break the normal use case of starting git-gui from within
    a .git subdirectory (where --show-toplevel would succeed).

  - A simple “non‑bare” check after a failed --show-toplevel would
    reject a normal repository whose worktree was only temporarily
    removed.

The chosen method keeps the original behavior for bare repositories and for regular working trees, fixes the crash, and properly blocks separated gitdirs without a reachable worktree.

Signed-off-by: Shroom Moo <egg_mushroomcow@foxmail.com>
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 git-gui/git-gui.sh | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)