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>