`magit-interactive-rebase': fully rely on `magit-guess-branch'

`magit-interactive-rebase' first tried to find a commit at point itself
before falling back `magit-guess-branch', which already provides that
functionality.  Also, if it *did* find a commit at point, it would
instantly (i.e. without confirmation) use that commit's first *parent*
as base, instead of the commit itself.

Now we no longer try to do `magit-guess-branch's job, and *always* ask
for confirmation.  Using the commit's first *parent* as base is of
course still possible, by completing the first match and then inputting
"^ [RET]".

Signed-off-by: Pieter Praet <pieter@praet.org>