object-name: reject trees found in the index

object-name: reject trees found in the index

The get_oid_with_context_1() method is used when parsing revision
arguments. One particular case is to take a ":<path>" string and search
the index for the given path.

In the case of a sparse index, this might find a sparse directory entry,
in which case the contained object is a tree. In the case of a full
index, this search within the index would fail.

In order to maintain identical return state as in a full index, inspect
the discovered cache entry to see if it is a sparse directory and reject
it. This requires being careful around the only_to_die option to be sure
we die only at the correct time.

This changes the behavior of 'git show :<sparse-dir>', but does not
bring it entirely into alignment with a full index case. It specifically
hits the wrong error message within diagnose_invalid_index_path(). That
error message will be corrected in a future change.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>