commit: drop duplicated parents

commit: drop duplicated parents

The scripted version of git-commit internally used git-commit-tree which
omitted duplicated parents given from the command line.  This prevented a
nonsensical octopus merge from getting created even when you said "git
merge A B" while you are already on branch A.

However, when git-commit was rewritten in C, this sanity check was lost.
This resurrects it.

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