rev-parse: integrate with sparse index

rev-parse: integrate with sparse index

It is not obvious that the 'git rev-parse' builtin would use the sparse
index, but it is possible to parse paths out of the index using the
":<path>" syntax. The 'git rev-parse' output is only the OID of the
object found at that location, but otherwise behaves similarly to 'git
show :<path>'. This includes the failure conditions on directories and
the error messages depending on whether a path is in the worktree or
not.

The only code change required is to change the
command_requires_full_index setting in builtin/rev-parse.c, and we can
re-use many existing 'git show' tests for the rev-parse case.

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