odb: rename git_odb_backend_malloc for consistency

The `git_odb_backend_malloc` name is a system function that is provided
for custom ODB backends and allows them to allocate memory for an ODB
object in the read callback.  This is important so that libgit2 can
later free the memory used by an ODB object that was read from the
custom backend.

However, the name _suggests_ that it actually allocates a
`git_odb_backend`.  It does not; rename it to make it clear that it
actually allocates backend _data_.

include/git2/sys/odb_backend.h

@@ -30,8 +30,8 @@ struct git_odb_backend {
 
 	/* read and read_prefix each return to libgit2 a buffer which
 	 * will be freed later. The buffer should be allocated using
-	 * the function git_odb_backend_malloc to ensure that it can
-	 * be safely freed later. */
+	 * the function git_odb_backend_data_alloc to ensure that libgit2
+	 * can safely free it later. */
 	int GIT_CALLBACK(read)(
 		void **, size_t *, git_object_t *, git_odb_backend *, const git_oid *);
 
@@ -117,8 +117,41 @@ GIT_EXTERN(int) git_odb_init_backend(
 	git_odb_backend *backend,
 	unsigned int version);
 
+/**
+ * Allocate data for an ODB object.  Custom ODB backends may use this
+ * to provide data back to the ODB from their read function.  This
+ * memory should not be freed once it is returned to libgit2.  If a
+ * custom ODB uses this function but encounters an error and does not
+ * return this data to libgit2, then they should use the corresponding
+ * git_odb_backend_data_free function.
+ *
+ * @param backend the ODB backend that is allocating this memory
+ * @param len the number of bytes to allocate
+ * @return the allocated buffer on success or NULL if out of memory
+ */
+GIT_EXTERN(void *) git_odb_backend_data_alloc(git_odb_backend *backend, size_t len);
+
+
+/*
+ * Users can avoid deprecated functions by defining `GIT_DEPRECATE_HARD`.
+ */
+#ifndef GIT_DEPRECATE_HARD
+
+/**
+ * Allocate memory for an ODB object from a custom backend.  This is
+ * an alias of `git_odb_backend_data_alloc` and is preserved for
+ * backward compatibility.
+ *
+ * This function is deprecated, but there is no plan to remove this
+ * function at this time.
+ *
+ * @deprecated git_odb_backend_data_alloc
+ * @see git_odb_backend_data_alloc
+ */
 GIT_EXTERN(void *) git_odb_backend_malloc(git_odb_backend *backend, size_t len);
 
+#endif
+
 GIT_END_DECL
 
 #endif

src/odb.c

@@ -1497,12 +1497,17 @@ int git_odb_write_pack(struct git_odb_writepack **out, git_odb *db, git_indexer_
 	return error;
 }
 
-void *git_odb_backend_malloc(git_odb_backend *backend, size_t len)
+void *git_odb_backend_data_alloc(git_odb_backend *backend, size_t len)
 {
 	GIT_UNUSED(backend);
 	return git__malloc(len);
 }
 
+void *git_odb_backend_malloc(git_odb_backend *backend, size_t len)
+{
+	return git_odb_backend_data_alloc(backend, len);
+}
+
 int git_odb_refresh(struct git_odb *db)
 {
 	size_t i;