SubmittingPatches: document how to retract a topic

SubmittingPatches: document how to retract a topic

While this document outlines an idealized lifecycle where an author
develops a patch, refines it with reviewer feedback, and
successfully merges it into Git, reality is rarely so seamless.

Sometimes, a topic must be abandoned. Doing so explicitly is far
better than leaving it in limbo, especially since topics can always
be resurrected later.

Clearly state that we encourage contributors to retract any topic
that does not pan out.

Signed-off-by: Junio C Hamano <redacted>