sparse-checkout: create helper methods

As we integrate the sparse index into more builtins, we occasionally
need to check the sparse-checkout patterns to see if a path is within
the sparse-checkout cone. Create some helper methods that help
initialize the patterns and check for pattern matching to make this
easier.

The existing callers of commands like get_sparse_checkout_patterns() use
a custom 'struct pattern_list' that is not necessarily the one in the
'struct index_state', so there are not many previous uses that could
adopt these helpers. There are just two in builtin/add.c and
sparse-index.c that can use path_in_sparse_checkout().

We add a path_in_cone_mode_sparse_checkout() as well that will only
return false if the path is outside of the sparse-checkout definition
_and_ the sparse-checkout patterns are in cone mode.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir.h b/dir.h
index b3e1a54a..6823312 100644
--- a/dir.h
+++ b/dir.h
@@ -394,6 +394,14 @@ enum pattern_match_result path_matches_pattern_list(const char *pathname,
 				const char *basename, int *dtype,
 				struct pattern_list *pl,
 				struct index_state *istate);
+
+int init_sparse_checkout_patterns(struct index_state *state);
+
+int path_in_sparse_checkout(const char *path,
+			    struct index_state *istate);
+int path_in_cone_mode_sparse_checkout(const char *path,
+				      struct index_state *istate);
+
 struct dir_entry *dir_add_ignored(struct dir_struct *dir,
 				  struct index_state *istate,
 				  const char *pathname, int len);