largefiles: download missing subrepo revs when archiving This is something plain hgsubrepos already do. Previously, an abort message complained about an unknown revision.

hgweb: change IE canvas test (issue3639) suggested by Peter Hull

store: optimize _pathencode by checking the length of the unencoded path If the input path is already longer than _maxstorepathlen, then we can skip doing the basic encoding (encodedir, _encodefname and _auxencode) and directly proceed to the hashed encoding. Those encodings, if at all, will make the path only longer.