fmt-merge-msg: use branch.$name.description

fmt-merge-msg: use branch.$name.description

This teaches "merge --log" and fmt-merge-msg to use branch description
information when merging a local topic branch into the mainline. The
description goes between the branch name label and the list of commit
titles.

The refactoring to share the common configuration parsing between
merge and fmt-merge-msg needs to be made into a separate patch.

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