When range of lines is a single line, the format differs from upstream #10

For the normal diff, when range of lines is a single line, the format differs from upstream.

(spotted when trying to run the upstream test suite, specifically that test)

Upstream compacts range "a,a" as just "a":

echo a > a
echo b > b
diff a b | head -1 # outputs "1c1"

Whereas this implementation doesn't compact it:

cargo run a b # incorrectly outputs "1,1c1,1"