Attempt to improve git-rebase lead-in description.

Attempt to improve git-rebase lead-in description.

It was mentioned on #git this morning that the lead-in description
of git-rebase is very confusing.  Too many branch this and branch
that in a very short run of text.

This new description attempts to walk the user through the command
syntax, while also describing exactly what git-rebase is doing to
their repository.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>