p7519: improve check for prerequisite WATCHMAN

p7519: improve check for prerequisite WATCHMAN

The return code of command -v with a non-existing command is 1 in bash
and 127 in dash.  Use that return code directly to allow the script to
work with dash and without watchman (e.g. on Debian).

While at it stop redirecting the output.  stderr is redirected to
/dev/null by test_lazy_prereq already, and stdout can actually be
useful -- the path of the found watchman executable is sent there, but
it's shown only if the script was run with --verbose.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Acked-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>