localrepo: make "undofiles()" return list of tuples "(vfs, relative filename)"
Before this patch, "localrepository.undofiles()" returns list of
absolute filename of undo files.
This patch makes it return list of tuples "(vfs, relative filename)"
to access undo files via vfs.
This patch also changes "repair.strip()", which is the only user of
"localrepository.undofiles()".