Use a typedef for the submodule_foreach callback.

This fits with the style for the rest of the project, but more
importantly, makes life easier for bindings authors who auto-generate
code.

include/git2/submodule.h

@@ -108,6 +108,18 @@ typedef enum {
 	GIT_SUBMODULE_STATUS_WD_UNTRACKED)) != 0)
 
 /**
+ * Function pointer to receive each submodule
+ *
+ * `sm` is the `git_submodule` currently being visited.
+ *
+ * `name` is the name of the submodule.
+ *
+ * `payload` is the value you passed to the foreach function as payload.
+ */
+typedef int (*git_submodule_cb)(
+	git_submodule *sm, const char *name, void *payload);
+
+/**
  * Submodule update options structure
  *
  * Use the GIT_SUBMODULE_UPDATE_OPTIONS_INIT to get the default settings,
@@ -239,7 +251,7 @@ GIT_EXTERN(void) git_submodule_free(git_submodule *submodule);
  */
 GIT_EXTERN(int) git_submodule_foreach(
 	git_repository *repo,
-	int (*callback)(git_submodule *sm, const char *name, void *payload),
+	git_submodule_cb callback,
 	void *payload);
 
 /**

src/submodule.c

@@ -495,7 +495,7 @@ cleanup:
 
 int git_submodule_foreach(
 	git_repository *repo,
-	int (*callback)(git_submodule *sm, const char *name, void *payload),
+	git_submodule_cb callback,
 	void *payload)
 {
 	git_vector snapshot = GIT_VECTOR_INIT;