status: show the branch name if possible in in-progress info

status: show the branch name if possible in in-progress info

The typical use-case is starting a rebase, do something else, come
back the day after, run "git status" or make a new commit and wonder
what in the world's going on. Which branch is being rebased is
probably the most useful tidbit to help, but the target may help
too.

Ideally, I would have loved to see "rebasing master on
origin/master", but the target ref name is not stored during rebase,
so this patch writes "rebasing master on a78c8c98b" as a
half-measure to remind future users of that potential improvement.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>