Describe the A...B shortcuts for checkout and rebase [-i] which were
introduced in these commits:

619a64e ("checkout A...B" switches to the merge base between A and B, 2009-10-18)
61dfa1bb ("rebase --onto A...B" replays history on the merge base between A and B, 2009-11-20)
230a4566 (rebase -i: teach --onto A...B syntax, 2010-01-07)
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>