Fix combined diffs on merge commits

A bad automated refactoring led us to try and obtain the EditList for
a combined diff patch script.  These scripts don't have any edits,
because there is more than old image involved in each range.

Fix the code to rule out a combined diff format script and do not
access its EditList when creating the cache entry.

Change-Id: Idc81370ffb4b1c7bf65c525043bf6b21314a01cf
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed