Checkout: Unifiy const-ness of `opts` parameter

Since all 3 checkout APIs perform the same operation with the options,
all of them should use the same const-ness.

include/git2/checkout.h

@@ -272,7 +272,7 @@ GIT_EXTERN(int) git_checkout_head(
 GIT_EXTERN(int) git_checkout_index(
 	git_repository *repo,
 	git_index *index,
-	git_checkout_opts *opts);
+	const git_checkout_opts *opts);
 
 /**
  * Updates files in the index and working tree to match the content of the
@@ -288,7 +288,7 @@ GIT_EXTERN(int) git_checkout_index(
 GIT_EXTERN(int) git_checkout_tree(
 	git_repository *repo,
 	const git_object *treeish,
-	git_checkout_opts *opts);
+	const git_checkout_opts *opts);
 
 /** @} */
 GIT_END_DECL

src/checkout.c

@@ -2018,7 +2018,7 @@ cleanup:
 int git_checkout_index(
 	git_repository *repo,
 	git_index *index,
-	git_checkout_opts *opts)
+	const git_checkout_opts *opts)
 {
 	int error;
 	git_iterator *index_i;
@@ -2053,7 +2053,7 @@ int git_checkout_index(
 int git_checkout_tree(
 	git_repository *repo,
 	const git_object *treeish,
-	git_checkout_opts *opts)
+	const git_checkout_opts *opts)
 {
 	int error;
 	git_tree *tree = NULL;