feat(tazjin/emacs): Add magit-find-file-worktree function

This will look up a file in the current worktree of the git repository
enclosing `default-directory'.

In combination with project-find-file this lets me toggle between
switching to a file within a project, and within the whole depot.

Change-Id: Ie1011f10051fc2c4bd4279b0944a79c7edf92f3b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1838
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/emacs/config/functions.el

@@ -289,4 +289,11 @@
 
 (add-to-list 'project-find-functions #'find-depot-project)
 
+(defun magit-find-file-worktree ()
+  (interactive)
+  "Find a file in the current (ma)git worktree."
+  (magit-find-file--internal "{worktree}"
+                             (magit-read-file-from-rev "HEAD" "Find file")
+                             #'pop-to-buffer-same-window))
+
 (provide 'functions)