format-patch: introduce branch.*.forkedFrom

A very common workflow for preparing patches involves working off a
topic branch and generating patches against 'master' to send off to the
maintainer. However, a plain

  $ git format-patch -o outgoing

is a no-op on a topic branch, and the user has to remember to specify
'master' explicitly everytime. This problem is not unique to
format-patch; even a

  $ git rebase -i

is a no-op because the branch to rebase against isn't specified.

To tackle this problem, introduce branch.*.forkedFrom which can specify
the parent branch of a topic branch. Future patches will build
functionality around this new configuration variable.

Cc: Jeff King <peff@peff.net>
Cc: Junio C Hamano <gister@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>