thodg/libgit2

diff --git a/include/git2/index.h b/include/git2/index.h
index 1d91663..2a0b001 100644
--- a/include/git2/index.h
+++ b/include/git2/index.h
@@ -190,6 +190,38 @@ GIT_EXTERN(int) git_index_write(git_index *index);
  */
 GIT_EXTERN(int) git_index_read_tree(git_index *index, git_tree *tree);
 
+/**
+ * Write the index as a tree
+ *
+ * This method will scan the index and write a representation
+ * of its current state back to disk; it recursively creates
+ * tree objects for each of the subtrees stored in the index,
+ * but only returns the OID of the root tree. This is the OID
+ * that can be used e.g. to create a commit.
+ *
+ * The index instance cannot be bare, and needs to be associated
+ * to an existing repository.
+ *
+ * @param oid Pointer where to store the OID of the written tree
+ * @param index Index to write
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_write_tree(git_oid *oid, git_index *index);
+
+/**
+ * Write the index as a tree to the given repository
+ *
+ * This method will do the same as `git_index_write_tree`, but
+ * letting the user choose the repository where the tree will
+ * be written.
+ *
+ * @param oid Pointer where to store OID of the the written tree
+ * @param index Index to write
+ * @param repo Repository where to write the tree
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_index_write_tree_to(git_oid *oid, git_index *index, git_repository *repo);
+
 /**@}*/
 
 /** @name Raw Index Entry Functions
diff --git a/include/git2/tree.h b/include/git2/tree.h
index 2ee1f4a..527f818 100644
--- a/include/git2/tree.h
+++ b/include/git2/tree.h
@@ -185,24 +185,6 @@ GIT_EXTERN(int) git_tree_entry_to_object(
 	const git_tree_entry *entry);
 
 /**
- * Write a tree to the ODB from the index file
- *
- * This method will scan the index and write a representation
- * of its current state back to disk; it recursively creates
- * tree objects for each of the subtrees stored in the index,
- * but only returns the OID of the root tree. This is the OID
- * that can be used e.g. to create a commit.
- *
- * The index instance cannot be bare, and needs to be associated
- * to an existing repository.
- *
- * @param oid Pointer where to store the written tree
- * @param index Index to write
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_tree_create_fromindex(git_oid *oid, git_index *index);
-
-/**
  * Create a new tree builder.
  *
  * The tree builder can be used to create or modify
diff --git a/src/index.c b/src/index.c
index 8f684a3..73fd40a 100644
--- a/src/index.c
+++ b/src/index.c
@@ -264,8 +264,14 @@ int git_index_open(git_index **index_out, const char *index_path)
 	index = git__calloc(1, sizeof(git_index));
 	GITERR_CHECK_ALLOC(index);
 
-	index->index_file_path = git__strdup(index_path);
-	GITERR_CHECK_ALLOC(index->index_file_path);
+	if (index_path != NULL) {
+		index->index_file_path = git__strdup(index_path);
+		GITERR_CHECK_ALLOC(index->index_file_path);
+
+		/* Check if index file is stored on disk already */
+		if (git_path_exists(index->index_file_path) == true)
+			index->on_disk = 1;
+	}
 
 	if (git_vector_init(&index->entries, 32, index_cmp) < 0)
 		return -1;
@@ -275,13 +281,10 @@ int git_index_open(git_index **index_out, const char *index_path)
 	index->entries_search_path = index_srch_path;
 	index->reuc_search = reuc_srch;
 
-	/* Check if index file is stored on disk already */
-	if (git_path_exists(index->index_file_path) == true)
-		index->on_disk = 1;
-
 	*index_out = index;
 	GIT_REFCOUNT_INC(index);
-	return git_index_read(index);
+
+	return (index_path != NULL) ? git_index_read(index) : 0;
 }
 
 static void index_free(git_index *index)
@@ -394,7 +397,11 @@ int git_index_read(git_index *index)
 	git_buf buffer = GIT_BUF_INIT;
 	git_futils_filestamp stamp;
 
-	assert(index->index_file_path);
+	if (!index->index_file_path) {
+		giterr_set(GITERR_INDEX,
+			"Failed to read index: The index is in-memory only");
+		return -1;
+	}
 
 	if (!index->on_disk || git_path_exists(index->index_file_path) == false) {
 		git_index_clear(index);
@@ -426,6 +433,12 @@ int git_index_write(git_index *index)
 	struct stat indexst;
 	int error;
 
+	if (!index->index_file_path) {
+		giterr_set(GITERR_INDEX,
+			"Failed to write index: The index is in-memory only");
+		return -1;
+	}
+
 	git_vector_sort(&index->entries);
 	git_vector_sort(&index->reuc);
 
@@ -449,6 +462,29 @@ int git_index_write(git_index *index)
 	return 0;
 }
 
+int git_index_write_tree(git_oid *oid, git_index *index)
+{
+	git_repository *repo;
+
+	assert(oid && index);
+
+	repo = (git_repository *)GIT_REFCOUNT_OWNER(index);
+
+	if (repo == NULL) {
+		giterr_set(GITERR_INDEX, "Failed to write tree. "
+		  "The index file is not backed up by an existing repository");
+		return -1
+	}
+
+	return git_tree__write_index(oid, index, repo);
+}
+
+int git_index_write_tree_to(git_oid *oid, git_index *index, git_repository *repo)
+{
+	assert(oid && index && repo);
+	return git_tree__write_index(oid, index, repo);
+}
+
 unsigned int git_index_entrycount(git_index *index)
 {
 	assert(index);
diff --git a/src/tree.c b/src/tree.c
index 9ecefbb..dd9f948 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -491,16 +491,12 @@ on_error:
 	return -1;
 }
 
-int git_tree_create_fromindex(git_oid *oid, git_index *index)
+int git_tree__write_index(git_oid *oid, git_index *index, git_repository *repo)
 {
 	int ret;
 	git_repository *repo;
 
-	repo = (git_repository *)GIT_REFCOUNT_OWNER(index);
-
-	if (repo == NULL)
-		return tree_error("Failed to create tree. "
-		  "The index file is not backed up by an existing repository");
+	assert(oid && index && repo);
 
 	if (index->tree != NULL && index->tree->entries >= 0) {
 		git_oid_cpy(oid, &index->tree->oid);
diff --git a/src/tree.h b/src/tree.h
index 24b517c..b67c552 100644
--- a/src/tree.h
+++ b/src/tree.h
@@ -47,6 +47,12 @@ int git_tree__parse(git_tree *tree, git_odb_object *obj);
  */
 int git_tree__prefix_position(git_tree *tree, const char *prefix);
 
+
+/**
+ * Write a tree to the given repository
+ */
+int git_tree__write_index(git_oid *oid, git_index *index, git_repository *repo);
+
 /**
  * Obsolete mode kept for compatibility reasons
  */