largefiles: check unknown files with case awareness of the filesystem
Before this patch, largefiles extension checks unknown files in the
working directory always case sensitively.
This causes failure in updating from the revision X consisting of
'.hglf/A' (and "A" implicitly) to the revision Y consisting of 'a'
(not ".hglf/A") on case insensitive filesystem, because "A" in the
working directory is treated as colliding against and different from
'a' on the revision Y.
This patch uses "repo.dirstate.normalize()" to check unknown files
with case awareness of the filesystem.