I had previously tried to implement a fix for some problems I had

experienced using rbt post -X, initially pushed as 5f1b920.

Unfortunately, this fix caused some regressions, including a couple

broken unit tests.
Since then, I've narrowed the problem down further to using -X

specifically inside of git worktrees. This indicated to me the actual

bug, which is that GitClient._get_root_directory() was using a

somewhat bogus way of determining the toplevel root of the working tree.

I've changed that to use --show-toplevel instead of --git-dir, which

fixes everything.

Ran unit tests.
Ran rbt diff -X within a git worktree and saw that files were

  correctly excluded.