Preserve leading whitespace for wrapped list items.

The previous implementation indented subsequent wrapped lines by
transforming the entire initial prefix into spaces.
This implementation detects the leading whitespace present in the
initial prefix, and uses the same whitespace (be it tabs, spaces, or
some combination thereof) to indent subsequent lines.
The non-whitespace portion of the initial prefix is still transformed
into spaces as before.

wrap_plus.py

@@ -544,7 +544,9 @@ def _extract_prefix(self, paragraph_r, lines, required_comment_prefix):
         m = list_pattern.match(first_line)
         if m:
             initial_prefix = first_line[0:m.end()]
-            subsequent_prefix = ' '*self._width_in_spaces(initial_prefix)
+            stripped_prefix = initial_prefix.lstrip()
+            leading_whitespace = initial_prefix[:len(initial_prefix)-len(stripped_prefix)]
+            subsequent_prefix = leading_whitespace+' '*self._width_in_spaces(stripped_prefix)
         else:
             m = field_pattern.match(first_line)
             if m: