[PATCH v6 0/3] git-gui: robustify startup and fix environment handling

This series addresses the startup crash introduced by Git commit "2d92ab32fd ("rev-parse: make --show-toplevel without a worktree an error", 2019-11-19)", which causes `git gui` to die with a Tcl error when a worktree is missing (e.g. inside a .git directory without a working tree, or in a bare repository).

Additionally, it resolves two historically inconsistent behaviours:
- The "Visualize ... History" menu items were enabled in bare
  repositories but triggered Tcl errors due to the missing worktree.
- `GIT_WORK_TREE` and `GIT_DIR` environment variables were not
  respected early enough, so `GIT_WORK_TREE=/some/path git gui`
  often ignored the explicit worktree and brought up the repository
  picker, and an exported empty `GIT_WORK_TREE` confused commands
  like `git branch --show-current` in bare repositories.

Shroom Moo (3):
  git-gui: restructure repository startup
  git-gui: disable gitk visualization when no worktree available
  git-gui: handle GIT_DIR and GIT_WORK_TREE early

 git-gui/git-gui.sh | 170 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 128 insertions(+), 42 deletions(-)