Blame "linenr" link jumps to previous state at "orig_lineno"

Blame currently displays the commit id which introduced a
block of one or more lines, the line numbers wrt the current
listing of the file and the file's line contents.

The commit id displayed is hyperlinked to the commit.

Currently the linenr links are hyperlinked to the same
commit id displayed to the left, which is _no_ different
than the block of lines displayed, since it is the _same
commit_ that is hyperlinked.  And thus clicking on it leads
to the same state of the file for that chunk of
lines. I.e. data mining is not currently possible with
gitweb given a chunk of lines introduced by a commit.

This patch makes such data mining possible.

The line numbers are now hyperlinked to the parent of the
commit id of the block of lines.  Furthermore they are
linked to the line where that block was introduced.

Thus clicking on a linenr link will show you the file's
line(s) state prior to the commit id you were viewing.

So clicking continually on a linenr link shows you how this
line and its line number changed over time, leading to the
initial commit where it was first introduced.

Signed-off-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -3212,9 +3212,14 @@ HTML
 				      esc_html($rev));
 			print "</td>\n";
 		}
+		open (my $dd, "-|", git_cmd(), "rev-parse", "$full_rev^")
+			or die_error("could not open git-rev-parse");
+		my $parent_commit = <$dd>;
+		close $dd;
+		chomp($parent_commit);
 		my $blamed = href(action => 'blame',
 				  file_name => $meta->{'filename'},
-				  hash_base => $full_rev);
+				  hash_base => $parent_commit);
 		print "<td class=\"linenr\">";
 		print $cgi->a({ -href => "$blamed#l$orig_lineno",
 				-id => "l$lineno",