diff: when diffing a revision with a deleted subrepo, maintain the node context (
issue3153)
phases: add a function to compute visible heads
This function will be used to hide secret changeset.
phases: fix advanceboundary behavior when targetphase !=0
Changeset was properly removed from upper phase but was not explicitly added to
target phase. If advanced changesets had not pwasnt in target phase they was
considered public (0-phase).