Fix t3409.5

The idea was to test whether the rebase of M onto A2 (while preserving
the merge commits) in this graph works:

	A1 - B1 - M
	   \    /
	     A2

The original problem fixed in 12bf8283(rebase -i -p: include
non-first-parent commits in todo list) was that B1 was dropped.

However, the test was overzealous in assuming that M needs to be
preserved: when rebasing, in effect, B1 and M onto A2, B1 should
actually be cherry-picked, and M no longer be necessary. This is the
behavior of the upcoming new 'rebase -i -p'.

So let's test that the relevant commits are all there, but skip the test
whether the merge is still there (that would need to be a forced
non-fast-forward merge if B1 is rebased).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! rebase -i: introduce the 'merge' command
fixup! Teach rebase about preserving merges while in interactive mode
WIP: rebase -p: Add an option to rebase all merged branches

When rebasing preserving merges, we exclude topic branches from being
rebased when they spawned off from the upstream earlier than the current
branch. The newly introduced option --rebase-merged-branches forces
those branches to be rebased onto the upstream commit, too.

Example:

	A - B - C
          \       \
            D - E - F
	      \
	        G

When rebasing F onto G, preserving merges, the result will look like
this *without* --rebase-merged-branches:

	A - B - C ----
	  \           \
	    D - G - E - F'

It will look like this *with* --rebase-merged-branches:

	A - D - G - B' - C'
	          \        \
	            E' ----- F''

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>