really reset walker with git_revwalk_reset

From the description  of git_revwalk_reset in revwalk.h the function should
clear all pushed and hidden commits, and leave the walker in a blank state (just like at creation).
Apparently everything gets reseted appart of pushed commits (walk->one and walk->twos)

This fix should reset the walker properly.

src/revwalk.c

@@ -838,5 +838,8 @@ void git_revwalk_reset(git_revwalk *walk)
 	commit_list_free(&walk->iterator_rand);
 	commit_list_free(&walk->iterator_reverse);
 	walk->walking = 0;
+
+	walk->one = NULL;
+	git_vector_clear(&walk->twos);
 }