Description of problem:
Translation history has two "Compare ver. x and y" methods, which point to the same place.

Version-Release number of selected component (if applicable):
Dev

Steps to Reproduce:
1. Alter a translation target twice
2. Press the History icon
3. Press Compare on both items in history

Actual results:
Both the button and the tab text change to say "Compare x and y" but they both point to the same place - the tab.

Expected results:
There's no need to have a button, the tab is enough. 
It would be nicer if the tab changed to say "Compare ver. 1 and ?" on the first click, just to show the user they're doing the right thing.