hook.c: add a hook_exists() wrapper and use it in bugreport.c

hook.c: add a hook_exists() wrapper and use it in bugreport.c

Add a boolean version of the find_hook() function for those callers
who are only interested in checking whether the hook exists, not what
the path to it is.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>