sparse-index: create expand_index()

sparse-index: create expand_index()

This is the first change in a series to allow modifying the
sparse-checkout pattern set without expanding a sparse index to a full
one in the process. Here, we focus on the problem of expanding the
pattern set through a command like 'git sparse-checkout add <path>'
which needs to create new index entries for the paths now being written
to the worktree.

To achieve this, we need to be able to replace sparse directory entries
with their contained files and subdirectories. Once this is complete,
other code paths can discover those cache entries and write the
corresponding files to disk before committing the index.

We already have logic in ensure_full_index() that expands the index
entries, so we will use that as our base. Create a new method,
expand_index(), which takes a pattern list, but for now mostly ignores
it. The current implementation is only correct when the pattern list is
NULL as that does the same as ensure_full_index(). In fact,
ensure_full_index() is converted to a shim over expand_index().

A future update will actually implement expand_index() to its full
capabilities. For now, it is created and documented.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>