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>

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