When displaying no difference between files the server crashed
trying to guess the MIME type of the old and new image. If we
have no difference then there is no FileHeader returned in the
DiffCacheContent, so we cannot ask it for the file name to give
to the MIME type guessing routines. Instead we must rely upon
the DiffCacheKey, which also has the file names stored in it.
Bug: GERRIT-248
Signed-off-by: Shawn O. Pearce <sop@google.com>
aaa09300d8