t3427: fix another incorrect assumption

t3427: fix another incorrect assumption

The test case that concerns `git rebase -Xsubtree` (with the
default rebase backend, not with `--preserve-merges`) starts out with a
pre-rebase commit history that begins with a commit that introduces
three files: master1.t, master2.t and master3.t.

This commit was generated by passing a subtree merge commit through `git
filter-branch --subdirectory-filter`, so it looks as if this commit
really introduces all those files.

The commit history onto which this commit is then rebased, however,
introduced those files in individual commits. For that reason, the
rebase will fail, it _must_ fail, because the first `pick` results in no
changes to be committed.

Let's fix the test case to expect exactly this situation.

With this change, we can mark the original bug that this test case tried
to demonstrate as fixed.

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