Make "stg push" subdirectory safe

Make "stg push" 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 push subroutine should instead be safe to run
from a subdirectory. However, pushing from a subdirectory currently
erases the parts of a patch that doesn't touch that subdirectory, and
that has to be fixed.

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