SubmittingPatchs: clarify choice of base and testing

SubmittingPatchs: clarify choice of base and testing

We encourage identifying what, among many topics on `next`, exact
topics a new work depends on, instead of building directly on
`next`.  Let's clarify this in the documentation.

Developers should know what they are building on top of, and be
aware of which part of the system is currently being worked on.
Encouraging them to make trial merges to `next` and `seen`
themselves will incentivize them to read others' changes and
understand them, eventually helping the developers to coordinate
among themselves and reviewing each others' changes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>