hg: Show all open branches.

This changes the hg backend to also include inactive branches i.e. branches that are merged but not closed.

https://github.com/rtfd/readthedocs.org/issues/398
Merge pull request #419 from djc/patch-1

Update Mercurial dependency to 2.6.3 (fixes #418).
Merge pull request #404 from lazka/hg-branches

hg: Show all open branches.
Update docs on hg and include syncdb command.
Use Persona for user accounts (fixes #359).