read-cache: move shared add/checkout/commit code

read-cache: move shared add/checkout/commit code

The function add_files_to_cache(), plus associated helper functions,
were defined in builtin/add.c, but also shared with builtin/checkout.c
and builtin/commit.c.  Move these shared functions to read-cache.c.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>