git:checkout-worktree: Fix exit when not in repo

Fix the exit condition when the function is called outside of a git
repository.
Use zsh's `:t` History Expansion Modifier, as `$?` does not keep `git
rev-parse`'s exit code, but rather `basename`'s.

.config/zsh/autoload/git/git-checkout-worktree

@@ -7,7 +7,7 @@
 # (e.g. stashing). Finally the temporary directory is deleted.
 
 local GIT_ROOT TEMP_DIR REPO_DIR
-GIT_ROOT="$(basename "$(git rev-parse --show-toplevel)")" || return
+GIT_ROOT="${$(git rev-parse --show-toplevel):t}" || return
 TEMP_DIR="$(mktemp -d)"
 REPO_DIR="$TEMP_DIR/$GIT_ROOT"