Teach rebase -i about --preserve-merges

The option "-p" (or long "--preserve-merges") makes it possible to
rebase side branches including merges, without straightening the
history.

Example:

           X
            \
         A---M---B
        /
---o---O---P---Q

When the current HEAD is "B", "git rebase -i -p --onto Q O" will yield

               X
                 \
---o---O---P---Q---A'---M'---B'

Note that this will

- _not_ touch X [*1*], it does

- _not_ work without the --interactive flag [*2*], it does

- _not_ guess the type of the merge, but blindly uses recursive or
  whatever strategy you provided with "-s <strategy>" for all merges it
  has to redo, and it does

- _not_ make use of the original merge commit via git-rerere.

*1*: only commits which reach a merge base between <upstream> and HEAD
     are reapplied. The others are kept as-are.

*2*: git-rebase without --interactive is inherently patch based (at
     least at the moment), and therefore merges cannot be preserved.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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