Get commands to raise Abort instead of ui.warn(...),sys.exit(1).

Add a source designator to the walk methods. If the source is 'f' (the filesystem), the file definitely exists in the filesystem. If 'm' (a rev or dirstate manifest), the file may not still exist with the given name.

Merge with Matt's tip.

Get add and locate to use new repo and dirstate walk code. They use a walk function that abstracts out the irritating details, so that there's a higher likelihood of commands behaving uniformly.