show-branch: --list and --independent

The --list option is what 'git branch' without parameter should
have been; it shows the one-line commit message for each branch
name.  The --independent option is used to filter out commits
that can be reachable from other commits, to make detection of
fast forward condition in multi-head merge easier.

Signed-off-by: Junio C Hamano <junkio@cox.net>