clone: const-ify checkout options

The removal of many options which lead to the direct usage of the
user's checkout options means we should make sure they remain const.

include/git2/checkout.h

@@ -255,7 +255,7 @@ typedef struct git_checkout_opts {
  */
 GIT_EXTERN(int) git_checkout_head(
 	git_repository *repo,
-	git_checkout_opts *opts);
+	const git_checkout_opts *opts);
 
 /**
  * Updates files in the working tree to match the content of the index.

include/git2/clone.h

@@ -104,7 +104,7 @@ GIT_EXTERN(int) git_clone(
  * default branch
  * @return 0 on success or an error code
  */
-GIT_EXTERN(int) git_clone_into(git_repository *repo, git_remote *remote, git_checkout_opts *co_opts, const char *branch);
+GIT_EXTERN(int) git_clone_into(git_repository *repo, git_remote *remote, const git_checkout_opts *co_opts, const char *branch);
 
 /** @} */
 GIT_END_DECL

src/checkout.c

@@ -1119,7 +1119,7 @@ static void checkout_data_clear(checkout_data *data)
 static int checkout_data_init(
 	checkout_data *data,
 	git_iterator *target,
-	git_checkout_opts *proposed)
+	const git_checkout_opts *proposed)
 {
 	int error = 0;
 	git_repository *repo = git_iterator_owner(target);
@@ -1229,7 +1229,7 @@ cleanup:
 
 int git_checkout_iterator(
 	git_iterator *target,
-	git_checkout_opts *opts)
+	const git_checkout_opts *opts)
 {
 	int error = 0;
 	git_iterator *baseline = NULL, *workdir = NULL;
@@ -1404,7 +1404,7 @@ int git_checkout_tree(
 
 int git_checkout_head(
 	git_repository *repo,
-	git_checkout_opts *opts)
+	const git_checkout_opts *opts)
 {
 	int error;
 	git_tree *head = NULL;

src/checkout.h

@@ -19,6 +19,6 @@
  */
 extern int git_checkout_iterator(
 	git_iterator *target,
-	git_checkout_opts *opts);
+	const git_checkout_opts *opts);
 
 #endif

src/clone.c

@@ -331,7 +331,7 @@ on_error:
 static bool should_checkout(
 	git_repository *repo,
 	bool is_bare,
-	git_checkout_opts *opts)
+	const git_checkout_opts *opts)
 {
 	if (is_bare)
 		return false;
@@ -345,7 +345,7 @@ static bool should_checkout(
 	return !git_repository_head_unborn(repo);
 }
 
-int git_clone_into(git_repository *repo, git_remote *remote, git_checkout_opts *co_opts, const char *branch)
+int git_clone_into(git_repository *repo, git_remote *remote, const git_checkout_opts *co_opts, const char *branch)
 {
 	int error = 0, old_fetchhead;
 	size_t nspecs;