hook: fix minor style issues

hook: fix minor style issues

Fix some minor style nits pointed out by Patrick, Junio and Eric:
  * Use CALLOC_ARRAY instead of xcalloc.
  * Init struct members during declaration.
  * Simplify if condition boolean logic.
  * Missing curly braces in if/else stmts.
  * Unnecessary header includes.
  * Capitalization and full-stop in error/warn messages.
  * Curly brace on separate line when defining struct.
  * Comment spelling: free'd -> freed.
  * Sort the included headers.
  * Blank line fixes to improve readability.

These contain no logic changes, the code behaves the same as before.

Suggested-by: Eric Sunshine <sunshine@sunshineco.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>