revset: reject negative number to select first/last n members Negative 'lim' doesn't make sense here, and it makes things complicated when using list[:lim].

revset: fix order of last() n members where n > 1 (BC) last() is implemented using a reversed iterator, so the result should be reversed again. I've marked this as BC since it's quite old bug seen in 3.0. The first bad revision is 4849f574aa24 "revset: changed last implementation to use lazy classes."

debugrevspec: add option to suppress list of computed revisions Test will be added later.