Tooltips were added for the icons in the diff viewer beside each modified file
in ceb2b88.  During the code review at https://reviews.reviewboard.org/r/7880/
it was discussed that %d could not be used for as a format specifier as only %s
is supported.  But somehow, in the final revision all the existing instances
of %s were swapped out for %d.  This change restores the correct %s specifier.

Tested both before and after change.  Screenshots show incorrect initial
behavior, and desired behavior after change.