revwalk functions: return an int

Stop returning a void for functions, future-proofing them to allow them
to fail.

include/git2/revwalk.h

@@ -84,8 +84,9 @@ GIT_EXTERN(int) git_revwalk_new(git_revwalk **out, git_repository *repo);
  * is over.
  *
  * @param walker handle to reset.
+ * @return 0 or an error code
  */
-GIT_EXTERN(void) git_revwalk_reset(git_revwalk *walker);
+GIT_EXTERN(int) git_revwalk_reset(git_revwalk *walker);
 
 /**
  * Add a new root for the traversal
@@ -224,8 +225,9 @@ GIT_EXTERN(int) git_revwalk_next(git_oid *out, git_revwalk *walk);
  *
  * @param walk the walker being used for the traversal.
  * @param sort_mode combination of GIT_SORT_XXX flags
+ * @return 0 or an error code
  */
-GIT_EXTERN(void) git_revwalk_sorting(git_revwalk *walk, unsigned int sort_mode);
+GIT_EXTERN(int) git_revwalk_sorting(git_revwalk *walk, unsigned int sort_mode);
 
 /**
  * Push and hide the respective endpoints of the given range.
@@ -246,8 +248,10 @@ GIT_EXTERN(int) git_revwalk_push_range(git_revwalk *walk, const char *range);
  * Simplify the history by first-parent
  *
  * No parents other than the first for each commit will be enqueued.
+ *
+ * @return 0 or an error code
  */
-GIT_EXTERN(void) git_revwalk_simplify_first_parent(git_revwalk *walk);
+GIT_EXTERN(int) git_revwalk_simplify_first_parent(git_revwalk *walk);
 
 
 /**

src/revwalk.c

@@ -700,7 +700,7 @@ git_repository *git_revwalk_repository(git_revwalk *walk)
 	return walk->repo;
 }
 
-void git_revwalk_sorting(git_revwalk *walk, unsigned int sort_mode)
+int git_revwalk_sorting(git_revwalk *walk, unsigned int sort_mode)
 {
 	assert(walk);
 
@@ -719,11 +719,14 @@ void git_revwalk_sorting(git_revwalk *walk, unsigned int sort_mode)
 
 	if (walk->sorting != GIT_SORT_NONE)
 		walk->limited = 1;
+
+	return 0;
 }
 
-void git_revwalk_simplify_first_parent(git_revwalk *walk)
+int git_revwalk_simplify_first_parent(git_revwalk *walk)
 {
 	walk->first_parent = 1;
+	return 0;
 }
 
 int git_revwalk_next(git_oid *oid, git_revwalk *walk)
@@ -752,7 +755,7 @@ int git_revwalk_next(git_oid *oid, git_revwalk *walk)
 	return error;
 }
 
-void git_revwalk_reset(git_revwalk *walk)
+int git_revwalk_reset(git_revwalk *walk)
 {
 	git_commit_list_node *commit;
 
@@ -777,6 +780,8 @@ void git_revwalk_reset(git_revwalk *walk)
 	walk->limited = 0;
 	walk->did_push = walk->did_hide = 0;
 	walk->sorting = GIT_SORT_NONE;
+
+	return 0;
 }
 
 int git_revwalk_add_hide_cb(