setlocalversion: Use "grep -q" instead of piping output to "read dummy"

In some circumstances (eg when running a build in an emacs shell
buffer), I get a spew of messages like

    grep: writing output: Broken pipe

from setlocalversion, because the "read" subshell apparently exits as
soon as it reads one line and gives EPIPE to grep.  It's not clear to
me why this way of writing the check was used instead of just using
grep -q to suppress output, but unless there is some deep reason I
don't know, this way looks cleaner to me anyway, and gets rid of the
ugly message spew.

(I double checked at http://pubs.opengroup.org/onlinepubs/009604499/utilities/grep.html


and "grep -q" is specified in POSIX / SuS, so hopefully even people
cross-compiling the kernel on some bizarre host OS can't complain
about this change)

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>