[PATCH 0 of 1] Change cat command to use walk code

[Bryan O'Sullivan]

This brings the cat command up to date, so that it handles file names
in the same way as all other commands.

In addition, you can now do things like this:

  hg cat -o ../export/%p '**'

to get a poor man's export equivalent (it doesn't set the x bit on files).

	<b