Teach bash how to complete git-rebase.

Teach bash how to complete git-rebase.

As git-rebase is a popular command bash should know how to complete
reference names and its long options.  We only support completions
which make sense given the current state of the repository, that
way users don't get shown --continue/--skip/--abort on the first
execution.

Also added support for long option --strategy to git-merge, as I
missed that option earlier and just noticed it while implementing
git-rebase.

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