dirstate: avoid match.files() in walk()

match: introduce boolean prefix() method tl;dr: This is another step towards a (previously unstated) goal of eliminating match.files() in conditions. There are four types of matchers: * always: Matches everything, checked with always(), files() is empty * exact: Matches exact set of files, checked with isexact(), files() contains the files to match * patterns: Matches more complex patterns, checked with anypats(), files() contains roots of the matched patterns * prefix: Matches simple 'path:' patterns as prefixes ('foo' matches both 'foo' and 'foo/bar'), no single method to check, files() contains the prefixes to match For completeness, it would be nice to have a method for checking for the "prefix" type of matcher as well, so let's add that, making it return True simply when none of the others do. The larger goal here is to eliminate uses of match.files() in conditions (i.e. bool(match.files())). The reason for this is that there are scenarios when you would like to create a "prefix" matcher that happens to match no files. One example is for 'hg files -I foo bar'. The narrowmatcher also restricts the set of files given and it would not surprise me if have bugs caused by that already. Note that 'if m.files() and not m.anypats()' and similar is sometimes used to catch the "exact" and "prefix" cases above.