This modifies gettreediffline so that it when we get both a "U" line
and an "M" line for the same file in the output from git diff-files
or git diff-index --cached (used when the user clicks on a fake commit)
we don't add the same filename to the treediff list twice.
This also makes getblobdiffline recognize the "* Unmerged path ..."
lines we get when we ask for the actual diffs, and makes a tiny
optimization in makediffhdr.