Add forward and back buttons and make SHA1 IDs clickable links.

Add forward and back buttons and make SHA1 IDs clickable links.

When we display the commit message in the details pane, any string
of 40 [0-9a-f] characters that corresponds to a SHA1 ID that we
know about gets turned into a clickable link, and displayed in
blue and underlined.

We now keep a history of commits that we have looked at, and we
have forward and back buttons for moving within the history list.