Summary:
The computation of commit obsolescence is inconsistent. If we compute the full
set of obsolete commits in `mutation.obsoletecache.obsoletenodes`, then we
correctly ignore public commits as they cannot be obsolete.
However, if we compute the obsolescence state for a single public commit with
`mutation.obsoletecache.isobsolete`, and that commit somehow has a visible
successor, then we will incorrectly consider the commit as obsolete.
Similarly, `allpredecessors` and `allsuccessors` should stop when they hit a
public commit.
Reviewed By: quark-zju
Differential Revision: D20892778
fbshipit-source-id: 223cb8b2bc9f2f08124df6ff51c2eb208bb8eb5f
