unnecessary builtin commands are always loaded at startup

[Martin Pool]

In bzr 2.2b1, builtins.py has:

from bzrlib.cmd_version_info import cmd_version_info
from bzrlib.conflicts import cmd_resolve, cmd_conflicts, restore
from bzrlib.foreign import cmd_dpush
from bzrlib.sign_my_commits import cmd_sign_my_commits

these are fairly infrequently used, and loading every time can cause more disk io. They should be lazy.

[Martin Pool (mbp)]

mp https://code.edge.launchpad.net/~mbp/bzr/lazy-commands/+merge/22590