[PATCH 0 of 3] DAG creation utils

[Peter Arrenbrecht]

Improved and fixed version. doctests actually run now. Better file creation
options for debugbuilddag.

I thought I understood mpm wanting these commands in core, but I'm also
happy with moving them to an extension if that is preferred.

@mpm: The . and < commands are new. They do not change what we discussed
but provide alternatives. The . is handy for debugdag --dots when you
want to get a feel for a DAG's structure (as my old format did). The <
is nice when you want to go back to a fork point and then do +n so that
the new branch is exactly n long (with *fork+n you get n+1 nodes).

The ! and !! are also new and allow people to inject arbitrary shell
commands into revs.