branch heads: fix regression introduced in e67e5b60e55f (issue1726) For merge nodes it is not adequate to only check a single possible branch head for whether it is an ancestor of the latest head, but it needs to be done for each possible branch head.

subrepo: use hg.repository instead of creating localrepo directly this way, extensions' reposetup will be called, which allows for git subrepos to be handled by hg-git (and I believe the same goes for svn and hgsubversion)

ignore: separate pattern extraction from match compilation hgignore files have slightly different syntax from match objects, e.g. syntax: foo headers, regexp: forms, and re being relre. Put conversion from hgignore syntax into match syntax into a standalone function so that it is easier to validate hgignore hunks externally.