This fixes a bug where git finds the incorrect merge parent. Consider a
repository with trunk, branch1 of trunk, and branch2 of branch1.
Without this change, git interprets a merge of branch2 into trunk as a
merge of branch1 into trunk.
Signed-off-by: Steven Walter <stevenrwalter@gmail.com> Reviewed-by: Sam Vilain <sam@vilain.net> Signed-off-by: Eric Wong <normalperson@yhbt.net>