git-branch: add support for --merged and --no-merged

git-branch: add support for --merged and --no-merged

These options filter the output from git branch to only include branches
whose tip is either merged or not merged into HEAD.

The use-case for these options is when working with integration of branches
from many remotes: `git branch --no-merged -a` will show a nice list of merge
candidates while `git branch --merged -a` will show the progress of your
integration work.

Also, a plain `git branch --merged` is a quick way to find local branches
which you might want to delete.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>