feat!: Let 'easy::Object::try_into_…() return try_into::Error`. (#298)

That way, the typical usage of `try_into_commit()?` will not result
in a strange error about `Object` not being convertible into some
error. We think having a real error there is the least surprising.

git-repository/src/easy/object/commit.rs

@@ -21,6 +21,7 @@ mod error {
 pub use error::Error;
 
 use crate::bstr::BStr;
+use crate::easy;
 
 impl<'repo> Commit<'repo> {
     /// Parse the commits message into a [`MessageRef`][git_object::commit::MessageRef], after decoding the entire commit object.
@@ -46,10 +47,7 @@ impl<'repo> Commit<'repo> {
         let tree_id = self.tree_id().ok_or(Error::Decode)?;
         match self.handle.find_object(tree_id)?.try_into_tree() {
             Ok(tree) => Ok(tree),
-            Err(obj) => Err(Error::ObjectKind {
-                actual: obj.kind,
-                expected: git_object::Kind::Tree,
-            }),
+            Err(easy::object::try_into::Error { actual, expected, .. }) => Err(Error::ObjectKind { actual, expected }),
         }
     }
 

git-repository/src/easy/object/mod.rs

@@ -21,6 +21,18 @@ pub mod peel;
 ///
 pub mod tree;
 
+///
+pub mod try_into {
+    #[derive(thiserror::Error, Debug)]
+    #[allow(missing_docs)]
+    #[error("Object named {id} was supposed to be of kind {expected}, but was kind {actual}.")]
+    pub struct Error {
+        pub actual: git_object::Kind,
+        pub expected: git_object::Kind,
+        pub id: git_hash::ObjectId,
+    }
+}
+
 impl DetachedObject {
     /// Infuse this owned object with an [`easy::Handle`].
     pub fn attach(self, handle: &easy::Handle) -> Object<'_> {
@@ -60,13 +72,21 @@ impl<'repo> Object<'repo> {
     }
 
     /// Transform this object into a commit, or return it as part of the `Err` if it is no commit.
-    pub fn try_into_commit(self) -> Result<Commit<'repo>, Self> {
-        self.try_into()
+    pub fn try_into_commit(self) -> Result<Commit<'repo>, try_into::Error> {
+        self.try_into().map_err(|this: Self| try_into::Error {
+            id: this.id,
+            actual: this.kind,
+            expected: git_object::Kind::Commit,
+        })
     }
 
     /// Transform this object into a tree, or return it as part of the `Err` if it is no tree.
-    pub fn try_into_tree(self) -> Result<Tree<'repo>, Self> {
-        self.try_into()
+    pub fn try_into_tree(self) -> Result<Tree<'repo>, try_into::Error> {
+        self.try_into().map_err(|this: Self| try_into::Error {
+            id: this.id,
+            actual: this.kind,
+            expected: git_object::Kind::Tree,
+        })
     }
 }