gitweb: Mark commits with no "previous" in 'blame' view

gitweb: Mark commits with no "previous" in 'blame' view

Use "no-previous" class to mark blamed commits which do not have
"previous" header.  Those are commits in which blamed file was created
(added); this includes boundary commits.  This means that 'linenr'
link leads to blamed commit, not (one of) parent of blamed commit.
Therefore currently line number for such commit uses bold weight font
to denote this situation; the effect is subtle.

Use "multiple-previous" class in the opposite situation, where blamed
commit has multiple "previous" headers (is an evil merge).  Currently
this class is not used for styling.  In this situation 'linenr' link
leads to first of "previous" commits (first parent).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>