rebase --keep-base: imply --no-fork-point
Given the name of the option it is confusing if --keep-base actually
changes the base of the branch without --fork-point being explicitly
given on the command line.
The combination of --keep-base with an explicit --fork-point is still
supported even though --fork-point means we do not keep the same base
if the upstream branch has been rewound. We do this in case anyone is
relying on this behavior which is tested in t3431[1]
[1] https://lore.kernel.org/git/
20200715032014.GA10818@generichostname/
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>