Add explicit git_index ptr to diff and checkout

A number of diff APIs and the `git_checkout_index` API take a
`git_repository` object an operate on the index.  This updates
them to take a `git_index` pointer explicitly and only fall back
on the `git_repository` index if the index input is NULL.  This
makes it easier to operate on a temporary index.

include/git2/checkout.h

@@ -199,13 +199,15 @@ GIT_EXTERN(int) git_checkout_head(
 /**
  * Updates files in the working tree to match the content of the index.
  *
- * @param repo repository to check out (must be non-bare)
+ * @param repo repository into which to check out (must be non-bare)
+ * @param index index to be checked out (or NULL to use repository index)
  * @param opts specifies checkout options (may be NULL)
  * @return 0 on success, GIT_ERROR otherwise (use giterr_last for information
  * about the error)
  */
 GIT_EXTERN(int) git_checkout_index(
 	git_repository *repo,
+	git_index *index,
 	git_checkout_opts *opts);
 
 /**

include/git2/diff.h

@@ -339,12 +339,14 @@ GIT_EXTERN(int) git_diff_tree_to_tree(
  * @param diff Output pointer to a git_diff_list pointer to be allocated.
  * @param repo The repository containing the tree and index.
  * @param old_tree A git_tree object to diff from.
+ * @param index The index to diff with; repo index used if NULL.
  * @param opts Structure with options to influence diff or NULL for defaults.
  */
 GIT_EXTERN(int) git_diff_index_to_tree(
 	git_diff_list **diff,
 	git_repository *repo,
 	git_tree *old_tree,
+	git_index *index,
 	const git_diff_options *opts); /**< can be NULL for defaults */
 
 /**
@@ -357,11 +359,13 @@ GIT_EXTERN(int) git_diff_index_to_tree(
  *
  * @param diff Output pointer to a git_diff_list pointer to be allocated.
  * @param repo The repository.
+ * @param index The index to diff from; repo index used if NULL.
  * @param opts Structure with options to influence diff or NULL for defaults.
  */
 GIT_EXTERN(int) git_diff_workdir_to_index(
 	git_diff_list **diff,
 	git_repository *repo,
+	git_index *index,
 	const git_diff_options *opts); /**< can be NULL for defaults */
 
 /**