bugreport: collect list of populated hooks

bugreport: collect list of populated hooks

Occasionally a failure a user is seeing may be related to a specific
hook which is being run, perhaps without the user realizing. While the
contents of hooks can be sensitive - containing user data or process
information specific to the user's organization - simply knowing that a
hook is being run at a certain stage can help us to understand whether
something is going wrong.

Without a definitive list of hook names within the code, we compile our
own list from the documentation. This is likely prone to bitrot, but
designing a single source of truth for acceptable hooks is too much
overhead for this small change to the bugreport tool.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>