Add a --parent flag to "stgit pick".

This is useful to pick changes from a non-stgit branch, that were
recorded as a merge commit, while giving control on the parent to
use as "bottom" for the patch (for some reason stgit selects the
first parent by default).

Incidentally, it can be used to pick changes between arbitrary
trees in the revision graph.  Since that can be useful as well,
I did not implement the is_parent check I originally meant to add.

Signed-off-by: Yann Dirson <ydirson@altern.org>