t4205 (log-pretty-formats): avoid using `sed`

t4205 (log-pretty-formats): avoid using `sed`

For testing truncated log messages 'commit_msg' function uses `sed` to
cut a message. On various platforms `sed` behaves differently and
results of its work depend on locales installed. So, avoid using `sed`.
Use predefined expected outputs instead of calculated ones.

Signed-off-by: Alexey Shumkin <redacted>
Signed-off-by: Junio C Hamano <redacted>