gitweb: bugfix: a.list formatting regression

gitweb: bugfix: a.list formatting regression

Fix regression introduced by
commit 17d07443188909ef5f8b8c24043cb6d9fef51bca.

"a.list" being "bold", makes a myriad of things shown by
gitweb in bold font-weight, which is a regression from
pre-17d07443188909ef5f8b8c24043cb6d9fef51bca behavior.

The fix is to add "subject" class and use this class
to replace pre-format_subject_html formatting of subject
(comment) via using (or not) <b>...</b> element. This
should go back to the pre-17d0744318... style.

Regression noticed by Luben Tuikov.

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