Make "stg refresh" subdirectory safe

Make "stg refresh" subdirectory safe by letting it internally cd up to
the top of the worktree. This is possibly not the best long-term fix;
one could argue that the refresh subroutine should instead be safe to
run from a subdirectory. However, refreshing from a subdirectory
currently only refreshes changes that are in the index, and not
changes in the working directory, and that has to be fixed.

Signed-off-by: Karl Hasselström <kha@treskal.com>