Make git_object_peel a bit smarter

This expands the types of peeling that `git_object_peel` knows
how to do to include TAG -> BLOB peeling, and makes the errors
slightly more consistent depending on the situation.  It also
adds a new special behavior where peeling to ANY will peel until
the object type changes (e.g. chases TAGs to a non-TAG).

Using this expanded peeling, this replaces peeling code that was
embedded in `git_tag_peel` and `git_reset`.

include/git2/object.h

@@ -168,11 +168,14 @@ GIT_EXTERN(int) git_object_typeisloose(git_otype type);
 GIT_EXTERN(size_t) git_object__size(git_otype type);
 
 /**
- * Recursively peel an object until an object of the specified
- * type is met
+ * Recursively peel an object until an object of the specified type is met.
  *
- * The retrieved `peeled` object is owned by the repository
- * and should be closed with the `git_object_free` method.
+ * The retrieved `peeled` object is owned by the repository and should be
+ * closed with the `git_object_free` method.
+ *
+ * If you pass `GIT_OBJ_ANY` as the target type, then the object will be
+ * peeled until the type changes (e.g. a tag will be chased until the
+ * referenced object is no longer a tag).
  *
  * @param peeled Pointer to the peeled git_object
  * @param object The object to be processed

include/git2/reset.h

@@ -37,7 +37,7 @@ GIT_BEGIN_DECL
  *
  * @return GIT_SUCCESS or an error code
  */
-GIT_EXTERN(int) git_reset(git_repository *repo, const git_object *target, git_reset_type reset_type);
+GIT_EXTERN(int) git_reset(git_repository *repo, git_object *target, git_reset_type reset_type);
 
 /** @} */
 GIT_END_DECL