git-web--browse: Fix check for /bin/start

git-web--browse: Fix check for /bin/start

The previous check in git-web--browse for /bin/start used test -n
/bin/start, which was always true.  This lead to "start" being tried
first in the browser list.  On systems with upstart installed, "start"
exists and might be in the PATH, but it makes a poor choice for a web
browser.  Instead, test that /bin/start exists and is executable.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>