GIT 0.99.9n aka 1.0rc6

GIT 0.99.9n aka 1.0rc6

Oh, I hate to do this but I ended up merging big usage string
cleanups from Fredrik, git-am enhancements that made a lot of
sense for non mbox users from HPA, and rebase changes (done
independently by me and Lukas) among other things, so git is
still in perpetual state of 1.0rc.  1.0 will probably be next
Wednesday, but who knows.

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