trace2: trim_trailing_newline followed by trim is a no-op

trace2: trim_trailing_newline followed by trim is a no-op

strbuf_trim_trailing_newline() removes a LF or a CRLF from the tail
of a string.  If the code plans to call strbuf_trim() immediately
after doing so, the code is better off skipping the EOL trimming in
the first place.  After all, LF/CRLF at the end is a mere special
case of whitespaces at the end of the string, which will be removed
by strbuf_rtrim() anyway.

Signed-off-by: Junio C Hamano <gitster@pobox.com>