rm: expand the index only when necessary

Remove the `ensure_full_index()` method so `git-rm` does not always expand the index when the expansion is unnecessary, i.e. when <pathspec> does not have any possibilities to match anything outside of sparse-checkout definition. Expand the index when the <pathspec> needs an expanded index, i.e. the <pathspec> contains wildcard that may need a full-index or the <pathspec> is simply outside of sparse-checkout definition. Notice that the test 'rm pathspec expands index when necessary' in t1092 *is* testing this code change behavior, though it will be marked as 'test_expect_success' only in the next patch, where we officially mark `command_requires_full_index = 0`, so the index does not expand unless we tell it to do so. Notice that because we also want `ensure_full_index` to record the stdout and stderr from Git command, a corresponding modification is also included in this patch. The reason we want the "sparse-index-out" and "sparse-index-err", is that we need to make sure there is no error from Git command itself, so we can rely on the `test_region` result and determine if the index is expanded or not. Helped-by: Victoria Dye <vdye@github.com> Helped-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>