Rename the revwalk constructor Walk() -> NewRevWalk()

This makes it be more in line with Go conventions and uses its full
name.
diff --git a/repository.go b/repository.go
index eec95dc..c2c8cec 100644
--- a/repository.go
+++ b/repository.go
@@ -154,7 +154,7 @@
 	return newReferenceFromC(ptr), nil
 }
 
-func (v *Repository) Walk() (*RevWalk, error) {
+func (v *Repository) NewRevWalk() (*RevWalk, error) {
 	walk := new(RevWalk)
 	ecode := C.git_revwalk_new(&walk.ptr, v.ptr)
 	if ecode < 0 {
diff --git a/walk_test.go b/walk_test.go
index 540d1a7..b50d626 100644
--- a/walk_test.go
+++ b/walk_test.go
@@ -10,7 +10,7 @@
 	defer os.RemoveAll(repo.Workdir())
 	commitId, _ := seedTestRepo(t, repo)
 
-	walk, err := repo.Walk()
+	walk, err := repo.NewRevWalk()
 	checkFatal(t, err)
 	walk.Push(commitId)
 	walk.Sorting(SortTime | SortReverse)