outgoing: fix common-heads computation from `missingroots` argument When initializing a `outgoing` object, the `common set` can be defined explicitly (with the `commonheads` argument`) or implicitly (with the missingroots arguments). It turns out the logic to compute `commonheads` from `missingroots` is buggy, as it does not consider the parents of enough changesets. Previously, it only considered parents of "missingroots` items, while it need to consider all parents of missing. Here is an example: F |\ C E | | B D |/ A If we use [E] as missing-roots, the missing set is [E, F], and the common-heads are [C, D]. However you cannot only consider the parent of [E] to find them, as [C] is not a parent of [E]. This already fix the bundle generated in one test, and it would prevent many other to misbehave with future change from this series.