feat: Object::peel_to_tree() as convenience method. (#470)

It's very common to try to work with trees, so let's make that easier.

git-repository/src/object/peel.rs

@@ -2,7 +2,7 @@
 use crate::{
     object,
     object::{peel, Kind},
-    Object,
+    Object, Tree,
 };
 
 ///
@@ -67,6 +67,11 @@ impl<'repo> Object<'repo> {
         }
     }
 
+    /// Peel this object into a tree and return it, if this is possible.
+    pub fn peel_to_tree(self) -> Result<Tree<'repo>, peel::to_kind::Error> {
+        Ok(self.peel_to_kind(git_object::Kind::Tree)?.into_tree())
+    }
+
     // TODO: tests
     /// Follow all tag object targets until a commit, tree or blob is reached.
     ///