gitweb: Print commit message without title in commitdiff only if there is any

gitweb: Print commit message without title in commitdiff only if there is any

Print the rest of commit message (title, i.e. first line of commit
message, is printed separately) only if there is any.

In repository which uses signoffs this shouldn't happen, because
commit message should consist of at least title and signoff.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>