git-svn: check_cherry_pick should exclude commits already in our history

git-svn: check_cherry_pick should exclude commits already in our history

The merge-base between @$parents and $merge_tip may have been reached
through a merge commit.  This means that some commits that are ancestors
of @$parents will not be ancestors of $merge_base.  The mergeinfo
property will not list commits that are ancestors of @$parents, so we
need to explicitly exclude them.

[ew: squashed and cleaned up test case from Steven]

Signed-off-by: Steven Walter <stevenrwalter@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>