t4055: avoid use of sed 'a' command

t4055: avoid use of sed 'a' command

The 'a', 'i' and 'c' commands take a literal text to be added
followed by backslash, but then in the source we cannot indent
the literal text which makes it ugly.

We need to also remember to double the backslash inside double
quotes.

Avoid these issues altogether by having an extra line in a template
file and generate test vectors by deleting the line or replacing the
line and not using the 'a' command.

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