t1092: test merge conflicts outside cone

t1092: test merge conflicts outside cone

Conflicts can occur outside of the sparse-checkout definition, and in
that case users might try to resolve the conflicts in several ways.
Document a few of these ways in a test. Make it clear that this behavior
is not necessarily the optimal flow, since users can become confused
when Git deletes these files from the worktree in later commands.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>