Documentation/git-rebase.txt: -f forces a rebase that would otherwise be a no-op

"Current branch is a descendant of the commit you are rebasing onto"
does not necessarily mean "rebase" requires "--force".  For a plain
vanilla "history flattening" rebase, the rebase can be done without
forcing if there is a merge between the tip of the branch being
rebased and the commit you are rebasing onto, even if the tip is
descendant of the other.

[jc: reworded both the text and the log description]

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>