zsh: Make git-checkout-worktree autoloadable

Transform `git-checkout-worktree` into an autoloadable function.

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

@@ -0,0 +1,30 @@
+#!/usr/bin/env zsh
+
+# Creates a git worktree checking it out the first argument in a temporary
+# directory that is deleted again, if the spawned subshell exits.
+
+local GIT_ROOT TEMP_DIR REPO_DIR
+GIT_ROOT="$(basename "$(git rev-parse --show-toplevel)")" || return
+TEMP_DIR="$(mktemp -d)"
+REPO_DIR="$TEMP_DIR/$GIT_ROOT"
+
+git worktree add "$REPO_DIR" "$1"
+[[ -e "$REPO_DIR" ]] || return 1
+pushd -q "$REPO_DIR"
+
+# Start subshell
+"$SHELL"
+
+# Cleanup when exiting
+popd -q
+
+# Restart the subshell until every issue is resolved and the worktree is
+# removed
+until [[ ! -e "$REPO_DIR" ]] || git worktree remove "$REPO_DIR"; do
+	pushd -q "$REPO_DIR"
+	"$SHELL"
+	popd -q
+done
+
+git worktree prune
+command rm -rf "$TEMP_DIR"