Previously, the collapse diff button on reviewable diffs and diffs in 

comment fragments could not be used with the keyboard as shown in the 

attached video collapse-btn-old.mov, unlike the expand diff button. This

review request makes the collapse diff button keyboard accessible. The new

behavior is documented in the attached video collapse-btn-new.mov. There

is also a side-by-side screenshot comparison of the old and new button.
The collapse button used to be a <div>, which is not focusable by 

default. The fix is to change the <div> to an <a>, which made the button 

focusable but did not invoke the collapse diff action. (Using an <a> is 

consistent with the expand diff button.) By adding the href=# attribute, 

both clicking on the element and using the enter key while the element is 

focused invokes the click handler to collapse the diff.

Manual testing on reviewable diffs and comment fragment diffs:

- used tab & shift+tab to check the collapse diff button can be tabbed to

- used enter key to invoke the collapse diff action and re-expand diff 

using only the keyboard