fetch/pull: use the sparse index

The 'git fetch' and 'git pull' commands parse the index in order to
determine if submodules exist. Without command_requires_full_index=0,
this will expand a sparse index, causing slow performance even when
there is no new data to fetch.

The .gitmodules file will never be inside a sparse directory entry, and
even if it was, the index_name_pos() method would expand the sparse
index if needed as we search for the path by name. These commands do not
iterate over the index, which is the typical thing we are careful about
when integrating with the sparse index.

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

builtin/fetch.c

@@ -1999,6 +1999,8 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	}
 
 	git_config(git_fetch_config, NULL);
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 
 	argc = parse_options(argc, argv, prefix,
 			     builtin_fetch_options, builtin_fetch_usage, 0);

builtin/pull.c

@@ -994,6 +994,8 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		set_reflog_message(argc, argv);
 
 	git_config(git_pull_config, NULL);
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 
 	argc = parse_options(argc, argv, prefix, pull_options, pull_usage, 0);
 

t/t1092-sparse-checkout-compatibility.sh

@@ -1009,6 +1009,16 @@ test_expect_success 'sparse index is not expanded: blame' '
 	done
 '
 
+test_expect_success 'sparse index is not expanded: fetch/pull' '
+	init_repos &&
+
+	git -C sparse-index remote add full "file://$(pwd)/full-checkout" &&
+	ensure_not_expanded fetch full &&
+	git -C full-checkout commit --allow-empty -m "for pull merge" &&
+	git -C sparse-index commit --allow-empty -m "for pull merge" &&
+	ensure_not_expanded pull full base
+'
+
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
 # in this scenario, but it shouldn't.
 test_expect_success 'reset mixed and checkout orphan' '