clone: add a server-side option to disable full getbundles (pull-based clones)
For large enough repositories, pull-based clones take too long, and an attempt
to use them indicates some sort of configuration or other issue or maybe an
outdated Mercurial. Add a config option to disable them.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])