t4205: unfold across multiple lines

Tests in t4205 test the following: git log --format='%(trailers:unfold)' ... By ensuring the multi-line trailers are unfolded back onto the same line. t4205 only includes tests for 2-line trailers, but `unfold()` will fail for folded trailers on 3 or more lines. In preparation for adding subsequent tests in t6300 that test similar behavior in `git-for-each-ref(1)`, let's harden t4205 (and make it consistent with the changes in t6300) by ensuring that 3 or more line folded trailers are unfolded correctly. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>