Updating to have this encode as configured

Was getting some stack traces when this was writing out UTF-8 git commit log content. Made this change locally to address.

src/gitchangelog/gitchangelog.py

@@ -1446,7 +1446,7 @@ def FileInsertAtFirstRegexMatch(filename, pattern, flags=0,
 
     def write_content(f, content):
         for content_line in content:
-            f.write(content_line)
+            f.write(content_line.encode(_prefered_encoding))
 
     def _wrapped(content):
         index = idx(_file_regex_match(filename, pattern, flags=flags))