hook: warn when hook.<friendly-name>.jobs is set

hook: warn when hook.<friendly-name>.jobs is set

Issue a warning when the user confuses the hook process and event
namespaces by setting hook.<friendly-name>.jobs.

Detect this by checking whether the name carrying .jobs also has
.command, .event, or .parallel configured.  Extract is_friendly_name()
as a helper for this check, to be reused by future per-event config
handling.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>