ldiff does not behave well with empty files #100

Impacted version

bundle exec ldiff --version
ldiff 1.5.1

Context

When performing a diff between a non-empty file and an empty file (could it be /dev/null), the ldiff output is very different than gnu diff and I would say ... wrong!

Repro

Let's take a file with 4 lines name four_lines:

cat -A four_lines
first$
second$
third$
last$

And now let's compare it with an empty file using unified format: bundle exec ldiff -u /dev/null four_lines

Current result

Here is what we get

--- /dev/null   2025-01-27 23:29:19.000000000 +0100
+++ four_lines  2025-01-28 08:43:54.000000000 +0100
@@ -1,3 +1,7 @@
\ No newline at end of file
+first
+second
+third
+fourth

Expected result

I would expect the same result as GNU diff:

--- /dev/null   2025-01-27 23:29:19.936649933 +0100
+++ four_lines  2025-01-28 08:43:54.620170046 +0100
@@ -0,0 +1,4 @@
+first
+second
+third
+fourth

The main issues I see ordered by ascending criticality:

• there should be no No newline at end of file notice
• there seems to be an off by 1 thing in the range, but I think the patch is still valid (need to confirm)
• the number of "new lines" is definitely not 7!!!

More

The example I used here is 0 vs 4 lines in unified format, but:

• actually all format seems impacted
• any non-zero value would work 0 vs 1, 0 vs 2, 0 vs 1000
• it could also be inverested 1 vs 0, 2 vs 0 & 1000 vs 0 (the result is less scary though)

I'll submit soon a "fake PR" with tests to demonstrate some failures.