Faster startup by moving code out of commands.py?

[Peter Arrenbrecht]

Just a wild idea we had at the sprint: Maybe we could speed up hg by
moving most of the code out of commands.py into separate files. Then
each command would only have to parse code that it actually needs. And
the command table could be filled with things that point to which file
to load and which function to call in it.

Would also help with structuring the stuff in commands.py a bit.

-parren