Use `grep &>/dev/null` instead of `grep -q` when operating on piped stdin.

`grep -q` may exit as soon as it finds a match; this is a good optimization
for when the input is a file.  However, if the input is the output of
another program, then that other program will receive SIGPIPE, and further
writes will fail.  When this happens, it might (bsdtar does) print a
message about a "write error" to stderr.  Which is going to confuse and
alarm the user.

I'll add that this is not purely hypothetical--it has happened to me while
running the test suite.

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