stash: teach 'push' (and 'create_stash') to honor pathspec

stash: teach 'push' (and 'create_stash') to honor pathspec

While working on a repository, it's often helpful to stash the changes
of a single or multiple files, and leave others alone.  Unfortunately
git currently offers no such option.  git stash -p can be used to work
around this, but it's often impractical when there are a lot of changes
over multiple files.

Allow 'git stash push' to take pathspec to specify which paths to stash.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>