sparse-checkout: extract pattern update from 'set' subcommand

In anticipation of adding "add" and "remove" subcommands to the
sparse-checkout builtin, extract a modify_pattern_list() method from the
sparse_checkout_set() method. This command will read input from the
command-line or stdin to construct a set of patterns, then modify the
existing sparse-checkout patterns after a successful update of the
working directory.

Currently, the only way to modify the patterns is to replace all of the
patterns. This will be extended in a later update.

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

builtin/sparse-checkout.c

@@ -462,29 +462,17 @@ static void add_patterns_from_input(struct pattern_list *pl,
 	}
 }
 
-static int sparse_checkout_set(int argc, const char **argv, const char *prefix)
+enum modify_type {
+	REPLACE,
+};
+
+static int modify_pattern_list(int argc, const char **argv, enum modify_type m)
 {
-	struct pattern_list pl;
 	int result;
 	int changed_config = 0;
-
+	struct pattern_list pl;
-	static struct option builtin_sparse_checkout_set_options[] = {
-		OPT_BOOL(0, "stdin", &set_opts.use_stdin,
-			 N_("read patterns from standard in")),
-		OPT_END(),
-	};
-
-	repo_read_index(the_repository);
-	require_clean_work_tree(the_repository,
-				N_("set sparse-checkout patterns"), NULL, 1, 0);
-
 	memset(&pl, 0, sizeof(pl));
 
-	argc = parse_options(argc, argv, prefix,
-			     builtin_sparse_checkout_set_options,
-			     builtin_sparse_checkout_set_usage,
-			     PARSE_OPT_KEEP_UNKNOWN);
-
 	add_patterns_from_input(&pl, argc, argv);
 
 	if (!core_apply_sparse_checkout) {
@@ -502,6 +490,26 @@ static int sparse_checkout_set(int argc, const char **argv, const char *prefix)
 	return result;
 }
 
+static int sparse_checkout_set(int argc, const char **argv, const char *prefix)
+{
+	static struct option builtin_sparse_checkout_set_options[] = {
+		OPT_BOOL(0, "stdin", &set_opts.use_stdin,
+			 N_("read patterns from standard in")),
+		OPT_END(),
+	};
+
+	repo_read_index(the_repository);
+	require_clean_work_tree(the_repository,
+				N_("set sparse-checkout patterns"), NULL, 1, 0);
+
+	argc = parse_options(argc, argv, prefix,
+			     builtin_sparse_checkout_set_options,
+			     builtin_sparse_checkout_set_usage,
+			     PARSE_OPT_KEEP_UNKNOWN);
+
+	return modify_pattern_list(argc, argv, REPLACE);
+}
+
 static int sparse_checkout_disable(int argc, const char **argv)
 {
 	struct pattern_list pl;