branch: teach --edit-description option

branch: teach --edit-description option

Using branch.$name.description as the configuration key, give users a
place to write about what the purpose of the branch is and things like
that, so that various subsystems, e.g. "push -s", "request-pull", and
"format-patch --cover-letter", can later be taught to use this
information.

The "-m" option similar to "commit/tag" is deliberately omitted, as the
whole point of branch description is about giving descriptive information
(the name of the branch itself is a better place for information that fits
on a single-line).

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