Select the files on which to report status.

With git_status_foreach_ext, this will control which changes get callbacks. With git_status_list_new, these will control which changes are included in the list.

• GIT_STATUS_SHOW_INDEX_AND_WORKDIR is the default. This roughly matches git status --porcelain regarding which files are included and in what order.
• GIT_STATUS_SHOW_INDEX_ONLY only gives status based on HEAD to index comparison, not looking at working directory changes.
• GIT_STATUS_SHOW_WORKDIR_ONLY only gives status based on index to working directory comparison, not comparing the index to the HEAD.