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

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 <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Acked-by: Emily Shaffer <redacted>
Signed-off-by: Junio C Hamano <redacted>