dxr-build: add ignore patterns involving the whole path

This adds support for ignore_patterns that take the whole path into
account.  Any ignore patterns that start with "/" will be matched against
the whole path relative to the root of the source tree, instead of just the
leaf name of the item.

Note that this doesn't do the right thing - it will happily match sub-
directories with a single "*".

For example, setting ignore pattern to ".hg /build/" will mean ignoring both 1) any files or directories named ".hg", and 2) a directory at the root of the repo named "build".  This, however, will not ignore "/widget/windows/build/" because even though the leaf name matches, it does not start at the root of the source tree.