status: fix nested sparse directory diff in sparse index

status: fix nested sparse directory diff in sparse index

Enable the 'recursive' diff option for the diff executed as part of 'git
status'. Without the 'recursive' enabled, 'git status' reports index
changes incorrectly when the following conditions were met:

* sparse index is enabled
* there is a difference between the index and HEAD in a file inside a
  *subdirectory* of a sparse directory
* the sparse directory index entry is *not* expanded in-core

Because it is not recursive by default, the diff in 'git status' reports
changes only at the level of files and directories that are immediate
children of a sparse directory, rather than recursing into directories with
changes to identify the modified file(s). As a result, 'git status' reports
the immediate subdirectory itself as "modified".

Example:

$ git init
$ mkdir -p sparse/sub
$ echo test >sparse/sub/foo
$ git add .
$ git commit -m "commit 1"
$ echo somethingelse >sparse/sub/foo
$ git add .
$ git commit -a -m "commit 2"
$ git sparse-checkout set --cone --sparse-index 'sparse'
$ git reset --soft HEAD~1
$ git status
On branch master
You are in a sparse checkout.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        modified:   sparse/sub

Enabling the 'recursive' diff option in 'wt_status_collect_changes_index'
corrects this issue by allowing the diff to recurse into subdirectories of
sparse directories to find modified files. Given the same repository setup
as the example above, the corrected result of `git status` is:

$ git status
On branch master
You are in a sparse checkout.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        modified:   sparse/sub/foo

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>