builtin/cat-file: mark 'git cat-file' sparse-index compatible

This change affects how 'git cat-file' works with the index when
specifying an object with the ":<path>" syntax (which will give file
contents from the index).

'git cat-file' expands a sparse index to a full index any time contents
are requested from the index by specifying an object with the ":<path>"
syntax. This is true even when the requested file is part of the sparse
index, and results in much slower 'git cat-file' operations when working
within the sparse index.

Mark 'git cat-file' as not needing a full index, so that you only pay
the cost of expanding the sparse index to a full index when you request
a file outside of the sparse index.

Add tests to ensure both that:
- 'git cat-file' returns the correct file contents whether or not the
  file is in the sparse index
- 'git cat-file' expands to the full index any time you request
  something outside of the sparse index

Signed-off-by: Kevin Lyles <klyles+github@epic.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -1047,6 +1047,9 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
 	if (batch.buffer_output < 0)
 		batch.buffer_output = batch.all_objects;
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	/* Return early if we're in batch mode? */
 	if (batch.enabled) {
 		if (opt_cw)

t/t1092-sparse-checkout-compatibility.sh

@@ -2358,4 +2358,40 @@ test_expect_success 'advice.sparseIndexExpanded' '
 	grep "The sparse index is expanding to a full index" err
 '
 
+test_expect_success 'cat-file -p' '
+	init_repos &&
+	echo "new content" >>full-checkout/deep/a &&
+	echo "new content" >>sparse-checkout/deep/a &&
+	echo "new content" >>sparse-index/deep/a &&
+	run_on_all git add deep/a &&
+
+	test_all_match git cat-file -p :deep/a &&
+	ensure_not_expanded cat-file -p :deep/a &&
+	test_all_match git cat-file -p :folder1/a &&
+	ensure_expanded cat-file -p :folder1/a
+'
+
+test_expect_success 'cat-file --batch' '
+	init_repos &&
+	echo "new content" >>full-checkout/deep/a &&
+	echo "new content" >>sparse-checkout/deep/a &&
+	echo "new content" >>sparse-index/deep/a &&
+	run_on_all git add deep/a &&
+
+	echo ":deep/a" >in &&
+	test_all_match git cat-file --batch <in &&
+	ensure_not_expanded cat-file --batch <in &&
+
+	echo ":folder1/a" >in &&
+	test_all_match git cat-file --batch <in &&
+	ensure_expanded cat-file --batch <in &&
+
+	cat >in <<-\EOF &&
+	:deep/a
+	:folder1/a
+	EOF
+	test_all_match git cat-file --batch <in &&
+	ensure_expanded cat-file --batch <in
+'
+
 test_done