bugfix: separar etapa de lectura y escritura de changelog

src/git_flow/changelog.py

@@ -54,7 +54,7 @@ class Changelog:
 
     def _prepend(self, content: str):
         if isfile(self.FILENAME):
-            with open(self.FILENAME, "rw") as f:
+            with open(self.FILENAME, "r") as f:
                 content += f.read()
-                f.seek(0)
+            with open(self.FILENAME, "w") as f:
                 f.write(content)