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>

builtin/rev-parse.c

@@ -723,6 +723,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 			prefix = setup_git_directory();
 			git_config(git_default_config, NULL);
 			did_repo_setup = 1;
+
+			prepare_repo_settings(the_repository);
+			the_repository->settings.command_requires_full_index = 0;
 		}
 
 		if (!strcmp(arg, "--")) {