archive: drop the leading '.' path component from the prefix (issue4634) Unix utilities like tar will happily prefix the files it packs with './', but annoyingly, Windows Explorer will not show these packed files when it opens the archive. Since there doesn't seem to be a point in including './' in the path names, just drop it. The default 'hg archive' prefix is the basename of the archive, so specifying '.' allows for that default to be disabled completely.