Make "stg push" subdirectory safe

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>