bug#68989: grep -m[2+] > /dev/null

[Grisha Levit]

This example file will require multiple reads:

$ printf '%07d\n' {0..98304} > txt

When limiting the match count to 1, the input offset is correctly restored
before grep exits,  no matter the output:

$ { grep -m1 . >/dev/zero; head -n1; } < txt
0000001
$ { grep -m1 . >/dev/null; head -n1; } < txt
0000001

If the count is 2 or higher, this also works, unless the output is being
discarded to /dev/null -- in this case not only is the input offset not
restored, but the entire file is consumed:

$ { grep -m2 . >/dev/zero; head -n1; } < txt
0000002
$ { grep -m2 . >/dev/null; head -n1; } < txt
$