Index: accept a tree for read_tree()

An index may not have an associated repository, so giving it an id in
that case is useless. Raise an error in that case and accept a Tree
object to make the function useful then.

docs/working-copy.rst

@@ -26,6 +26,14 @@ Custom entries::
    >>> entry = pygit2.IndexEntry('README.md', blob_id, blob_filemode)
    >>> repo.index.add(entry)
 
+The index fulfills a dual role as the in-memory representation of the
+index file and data structure which represents a flat list of a
+tree. You can use it independently of the index file, e.g.
+
+  >>> index = pygit2.Index()
+  >>> entry = pygit2.IndexEntry('README.md', blob_id, blob_filemode)
+  >>> index.add(entry)
+
 The Index type
 ====================
 

src/index.c

@@ -426,26 +426,46 @@ Index_remove(Index *self, PyObject *args)
 PyDoc_STRVAR(Index_read_tree__doc__,
   "read_tree(tree)\n"
   "\n"
-  "Update the index file from the tree identified by the given oid.");
+  "Update the index file from the specified tree. The tree can be a Tree object or an Oid.\n"
+  "Using an Oid is only possible if this index is associated with a repository");
 
 PyObject *
 Index_read_tree(Index *self, PyObject *value)
 {
     git_oid oid;
-    git_tree *tree;
-    int err;
+    git_tree *tree = NULL;
+    int err, need_free = 0;
     size_t len;
 
     len = py_oid_to_git_oid(value, &oid);
-    if (len == 0)
-        return NULL;
+    if (len == 0) {
+        Tree *py_tree;
+        if (!PyObject_TypeCheck(value, &TreeType)) {
+            return NULL;
+        }
 
-    err = git_tree_lookup_prefix(&tree, self->repo->repo, &oid, len);
-    if (err < 0)
-        return Error_set(err);
+        PyErr_Clear();
+        py_tree = (Tree *) value;
+        tree = py_tree->tree;
+    }
+
+    /*
+     * if the user passed in an id but we're not associated with a
+     * repo, we can't do anything
+     */
+    if (tree == NULL && self->repo == NULL) {
+        PyErr_SetString(PyExc_TypeError, "id given but no associated repository");
+        return NULL;
+    } else if (tree == NULL) {
+        need_free = 1;
+        err = git_tree_lookup_prefix(&tree, self->repo->repo, &oid, len);
+        if (err < 0)
+            return Error_set(err);
+    }
 
     err = git_index_read_tree(self->index, tree);
-    git_tree_free(tree);
+    if (need_free)
+        git_tree_free(tree);
     if (err < 0)
         return Error_set(err);