R1:a34cb4bb1212

Added button for showing/hiding copies in Diffusion history view

Summary:
By default, indirect events in Diffusion SVN history views were visible.
Now they are invisible, but the visiblity can be changed using a button
in the top right corner.

Test Plan:
Tested on multiple different files that the history is shown correctly after
the page is loaded, after the button is clicked once and after the button is
clicked the second time.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley…