Script should be able to pull its own diff and operate on this

Right now, checkiid.py operates on an input (from stdin) of a diff file. It should, in addition, construct this diff file and run on it. This way, we can use it as a standalone tool, as well as an hg hook.

This should probably be used in combination with a set of revisions switches, so that we can use it incrementally.

Start by reading checkiid.py and tracing how it consumes a diff from stdin. Determine how the script should construct its own diff and how revision switches support incremental use. Done means it works both as a standalone tool and as an hg hook while retaining stdin-based operation.

Written by the indexing model from the issue text.