Use list comprehension in hg status.

Attempt to yield names in sorted order when walking. This is an improvement in behaviour, but the walk and changes code still has some flaws that make sorted name presentation difficult: - changes returns tuples of names, instead of a sorted list of (name, status) pairs. - walk yields deleted names after all others.

Ensure that dirstate.walk only yields names once. Its predecessor code used to do this, and now it does, too.

Clean up walk and changes code to use normalised names properly. New function: commands.pathto returns the relative path from one path to another. For example, given foo/bar and baz/quux, it will return ../../baz/quux. This new function is used by the walk and status code to print relative paths correctly. New command: debugwalk exercises the walk code without doing anything more. hg.dirstate.walk now yields normalised names. For example, if you're in the baz directory and you ask it to walk ../foo/bar/.., it will yield names starting with foo/. As a result of this change, all of the other walk and changes methods in this module also return normalised names. The util.matcher function now normalises globs and path names, so that it will match normalised names properly. Finally, util.matcher uses the non-glob prefix of a glob to tell walk which directories to scan. Perviously, a glob like foo/* would scan everything, but only return matches for foo/*. Now, foo/* only scans under foo (using the globprefix function), which is much faster.