revert: simplify getting commit subject in format_todo()

revert: simplify getting commit subject in format_todo()

format_todo() calls get_message(), but uses only the subject line of
the commit message.  As a minor optimization, save work and
unnecessary memory allocations by using find_commit_subject() instead.
Also, remove the unnecessary check on cur->item->buffer: the
lookup_commit_reference() call in parse_insn_line() has already made
sure of this.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>