read-cache: convert post-index-change to use hook.h

Move the post-index-change hook away from run-command.h to and over to
the new hook.h library.

This removes the last direct user of "run_hook_ve()" outside of
run-command.c ("run_hook_le()" still uses it). So we can make the
function static now. A subsequent commit will remove this code
entirely when "run_hook_le()" itself goes away.

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

run-command.h

@@ -235,7 +235,6 @@ int run_command(struct child_process *);
  */
 LAST_ARG_MUST_BE_NULL
 int run_hook_le(const char *const *env, const char *name, ...);
-int run_hook_ve(const char *const *env, const char *name, va_list args);
 
 /*
  * Trigger an auto-gc