Existing trailers are extracted from the input message by looking for
a group of one or more lines that (i) are all trailers, or (ii) contains at
-least one Git-generated trailer and consists of at least 25% trailers.
+least one Git-generated or user-configured trailer and consists of at
+least 25% trailers.
The group must be preceded by one or more empty (or whitespace-only) lines.
The group must either be at the end of the message or be the last
non-whitespace lines before a line that starts with '---'. Such three