Stop creating an unwanted history entry on view diff.

In Firefox changing the url hash by setting window.location.hash creates
a history entry. This behaviour is different from chrome et al so
instead we now use window.location.replace to avoid the extra entry.

Firefox will no longer return to #index_header when hitting the back
button after having clicked view diff.

Testing Done:
Visited the diff viewer using View Diff in both Firefox and Chrome
before and after the fix. Chrome's behaviour is unchanged and Firefox
no longer has the additional unwanted history entry.

Reviewed at https://reviews.reviewboard.org/r/6414/