builtin/merge.c: reduce parents early

Instead of waiting until we record the parents of resulting merge, reduce
redundant parents (including our HEAD) immediately after reading them.

The change to t7602 illustrates the essence of the effect of this change.
The octopus merge strategy used to be fed with redundant commits only to
discard them as "up-to-date", but we no longer feed such redundant commits
to it and the affected test degenerates to a regular two-head merge.

And obviously the known-to-be-broken test in t6028 is now fixed.

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

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