First pieces of generic templating support

[Matt Mackall]

So I've set a goal of adding "generic templating", which means:

- it should be possible to use templating for all commands
- there should be some built-in styles like JSON, XML, and pickle
- it should be relatively painless to convert existing code
- performance impact should be minimal when not in use

At this point I've introduced the basic interface, added a simple debug
mode, and converted the status command. So we can now get output like
this:

$ hg st --config ui.formatdebug=1
status = {
    {'status': '?', 'char': 'branches.txt'},
    {'status': '?', 'char': 'hgext/clonecache.py'},
    {'status': '?', 'char': 'idcheck'},
    {'status': '?', 'char': 'p'},
    {'status': '?', 'char': 'status-daemon.txt'},
}

To get an idea of what's involved in adding templating support, see:

http://www.selenic.com/hg/rev/68007f0557de

The most important piece is:

-                ui.write(format % repo.pathto(f, cwd),
-                         label='status.' + state)
+                fm.startitem()
+                fm.write("status char", format, char,
+                         repo.pathto(f, cwd), label=label)

Basically, we've taken the old ui.write and replaced it with an fm.write
that gives each field a name. When no templating is happening, this
falls through to a normal ui.write. But when a templater is in use, we
ignore the format string and build data to pass to the templater.

Future steps:

- add support for hgweb-style templater
- add JSON/XML/etc templaters
- figure out a scheme for per-command --styles
- add global options --template and --style
- convert all the commands

Help wanted!

-- 
Mathematics is the supreme nostalgia of our time.