add: modify add_files_to_cache() to avoid globals

add: modify add_files_to_cache() to avoid globals

The function add_files_to_cache() is used by all three of builtin/{add,
checkout, commit}.c.  That suggests this is common library code, and
should be moved somewhere else, like read-cache.c.  However, the
function and its helpers made use of two global variables that made
straight code movement difficult:
  * the_index
  * include_sparse
The latter was perhaps more problematic since it was only accessible in
builtin/add.c but was still affecting builtin/checkout.c and
builtin/commit.c without this fact being very clear from the code.  I'm
not sure if the other two callers would want to add a `--sparse` flag
similar to add.c to get non-default behavior, but exposing this
dependence will help if we ever decide we do want to add such a flag.

Modify add_files_to_cache() and its helpers to accept the necessary
arguments instead of relying on globals.

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