git-stash: fix pushing stash with pathspec from subdir

git-stash: fix pushing stash with pathspec from subdir

The `git stash push` command recently gained the ability to get a
pathspec as its argument to only stash matching files. Calling this
command from a subdirectory does not work, though, as one of the first
things we do is changing to the top level directory without keeping
track of the prefix from which the command is being run.

Fix the shortcoming by storing the prefix previous to the call to
`cd_to_toplevel` and then subsequently using `git rev-parse --prefix` to
correctly resolve the pathspec. Add a test to catch future breakage of
this usecase.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>