Otherwise the fix will be overwritten by next _octicons.less update.

This is follow-up for 22b0dfbb35.

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.

Just use secondary menu instead custom ".head.meta", which simplifies code.

Also do not display repo URL action when we are in subdirectory or viewing a
file.

This is more inline with way GitHub looks like and feels much more natural and
in style with rest of the interface.

Instead using own ellipsis, uses Semantic UI fixed single line table which
effectively applies ellipsis to all overflowing table cells.

NOTE: File list cannot use colspan="2" for 1st "Last commit" elements,
otherwise layout breaks with fixed table.

This is because SHA1 label is used in many other places, not only inside
.repository container.

Semantic UI .icon 1em font-size has priority over .octicon 16px, resulting
octicons rendered at 14px font-size, which is not okay since Octicons are meant
to be shown sizes that are multiples of 16px.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942