UI: Display last-commit header without 2nd column

This uses a CSS trick making first th to be relative block with width equal to
first two columns, effectively working around inability to use colspan="2" on
first row that was breaking "fixed-layout" for tables.

Also use grey header for last-commit SHA1 tag.

public/less/_repository.less

@@ -146,6 +146,11 @@
 					padding-top: 8px;
 					padding-bottom: 5px;
 					font-weight: normal;
+					&:first-child {
+						display: block;
+						position: relative;
+						width: 325%;
+					}
 				}
 				.ui.avatar {
 					margin-bottom: 5px;