rebase: add --show-current-patch

rebase: add --show-current-patch

It is useful to see the full patch while resolving conflicts in a
rebase. The only way to do it now is

    less .git/rebase-*/patch

which could turn out to be a lot longer to type if you are in a
linked worktree, or not at top-dir. On top of that, an ordinary user
should not need to peek into .git directory. The new option is
provided to examine the patch.

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