Introduce 'section' as canonical spelling for 'repo.group'

The 'repo.' prefix should be reserved for repo-specific options, but
the option 'repo.group' must still be honored to stay backwards
compatible.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.css

@@ -429,7 +429,7 @@ table.diff td div.del {
 	text-align: right;
 }
 
-table.list td.repogroup {
+table.list td.reposection {
 	font-style: italic;
 	color: #888;
 }