configitems: register 'blackbox.maxsize' as an example of 'configbytes' This exercise the default value handling in 'configbytes'.

blackbox: minor code reordering The version declaration should come first in my opinion. This will help gather the command table with the config table.

configitems: add a devel warning for extensions items overiding core one We do not want such case to pass silently. In the future we'll likely have useful tool for an extension to alter the existing definition in core.

configitems: add an official API for extensions to register config item Extensions can have a 'configtable' mapping and use 'registrar.configitem(table)' to retrieve the registration function. This behave in the same way as the other way for extensions to register new items (commands, colors, etc).

configitems: extract the logic to build a registrar on any configtable Having the logic available independently from the mapping used is a necessary step toward extensions support.

obsolete: skip 'changectx' usage in unstable computation We simplify the unstable computation code, skipping the expensive creation of changectx object. We focus on efficient set operation and revnumber centric functions. In my mercurial development repository, this provides a 3x speedup to the function: before: 5.319 ms after: 1.844 ms repo details: total changesets: 40886 obsolete changesets: 7756 mutable (not obsolete): 293 unstable: 30

obsolete: provide a small function to retrieve all mutable revisions More obsolescence related algorithm focus on the mutable revision. We provide a tiny utility function to make it easy to access this set.