Fix ignore regression. An empty or non-existent hgignore file must return False (don't ignore) for everything.

Switch to new syntax for .hgignore files. Here is the new syntax, in summary. Trailing white space is dropped. The escape character is "\". Comments start with #. Empty lines are skipped. Lines can be of the following formats: syntax: regexp # defaults following lines to non-rooted regexps syntax: glob # defaults following lines to non-rooted globs re:pattern # non-rooted regular expression glob:pattern # non-rooted glob pattern # pattern of the current default type The default pattern type is regexp, which is completely backwards compatible with the old hgignore syntax. In the dirstate class, the ignore method has been reworked to be based on the util.matcher function, by way of a new dirstate.hgignore method.

Merge with myself.

Fix performance of dirstate.changes with ignored directories.