Fix read-tree merging more than 3 trees using 3-way merge

Fix read-tree merging more than 3 trees using 3-way merge

For multi-base merges, we allowed read-tree -m to take more than
three trees (the last two are our and their branches, and all the
earlier ones, typically one but potentially more, are used as the
merge base).  Unfortunately, the conversion done by commit 933bf40
broke this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>