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.
repoview: add _gethiddenblockers method

This is a standalone function that will provide the ability for extensions to
wrap.
repoview: use _gethiddenblockers in computehidden

No functionality has changed, since we've only extracted the code into its own
function. Now extensions can wrap _gethiddenblockers to provide their own
blocker without polluting bookmarks or local tags.
changegroup: remove unused variable caught by pyflakes

--HG--
branch : stable
shelve: remove unused variable caught by pyflakes

--HG--
branch : stable
tests: remove unused variables from test-pathencode.py caught by pyflakes

--HG--
branch : stable
merge with stable