grep invert-match return code

[Tavis Ormandy]

POSIX says `grep -v` will "Select lines not matching any of
the specified patterns.", and the return code should be 1
if "No lines were selected.", however this doesnt seem to be
true in GNU grep:

$ printf "one\ntwo\nthree\n" | grep -v one; echo $?
two
three
0
$ printf "one\ntwo\nthree\n" | grep -v four; echo $?
one
two
three
0

both return 0, "One or more lines were selected", even though this
doesnt seem to be true in the second example, as no lines were
selected.

Sorry if I have misinterpreted the documentation.

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