config: mark as sparse-index compatible

`git config --blob :.lfsconfig` (and similar :path lookups) triggers
ensure_full_index() in sparse checkouts, expanding the entire sparse
index to a full index. On large repositories this takes seconds even
though the command only needs to look up a single path.

The root cause has two parts:

1. `git config` uses RUN_SETUP_GENTLY and never calls repo_config(),
   so the sparse checkout globals (core.sparseCheckout,
   core.sparseCheckoutCone) are unset when the index is first read.

2. Without those globals, is_sparse_index_allowed() returns false,
   causing ensure_correct_sparsity() to call ensure_full_index()
   during do_read_index().

Fix this by loading repo config and setting command_requires_full_index
to 0 in cmd_config(). This is the standard opt-in pattern used by the
~30 other commands that are sparse-index compatible.

The repo_config() call is needed because RUN_SETUP_GENTLY does not
load the default config callbacks that populate the sparse checkout
globals. Other sparse-index-compatible commands use RUN_SETUP, which
handles this during setup_git_directory().

The flag is safe to set unconditionally because git config only
accesses the index when resolving --blob :path specs. For non-blob
invocations, the flag has no effect. For --blob :path, in-cone paths
resolve directly from the sparse index; out-of-cone paths trigger
on-demand expansion through index_name_pos() with EXPAND_SPARSE.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Author: spkrka

builtin/config.c

@@ -1618,6 +1618,12 @@ int cmd_config(int argc,
 	 */
 	argc = parse_options(argc, argv, prefix, subcommand_opts, builtin_config_usage,
 			     PARSE_OPT_SUBCOMMAND_OPTIONAL|PARSE_OPT_KEEP_ARGV0|PARSE_OPT_KEEP_UNKNOWN_OPT);
+	if (startup_info->have_repository) {
+		repo_config(repo, git_default_config, NULL);
+		prepare_repo_settings(repo);
+		repo->settings.command_requires_full_index = 0;
+	}
+
 	if (subcommand) {
 		argc = parse_options(argc, argv, prefix, subcommand_opts, builtin_config_usage,
 		       PARSE_OPT_SUBCOMMAND_OPTIONAL|PARSE_OPT_KEEP_UNKNOWN_OPT);