Support combined diffs
Support combined diffs

To support combined diffs, we just need some good enough start values
for counting.

Note that really counting a combined diff is beyond the scope of the
change here.  We just need a good sortable tuple (`(int, int)`) to
mark lines in a predictable, stable way.

divmain/GitSavvy,divmain/GitSavvy,divmain/GitSavvy