repoview: add _gethiddenblockers method
This is a standalone function that will provide the ability for extensions to
wrap.
repoview: improve performance for computehidden (
issue4206)
For repos with a large number of heads (including hidden heads), a stale tag
cache would cause computehidden to be drastically slower because of a the call
to repo.tags() (which would build the tag cache).
We actually don't need the tag cache for computehidden because we filter out
global tags. This patch replaces the call to repo.tags with readlocaltags so
as to avoid the tag cache.