Fix hunk/ldiff unified diff against empty files

The ldiff bin (and it's underlying implementation) behaves weirdly when
given an empty file as one of its operand (old or new file).

This is particularly painful when doing diff for new or deleted files,
especially as one could think it is a simple case as the whole content
should be considered either added or removed.

To demonstrate the issue, I'm adding test cases to demonstrate
how ldiff behaves when a 4-lines file is compared to empty, using all
supported formats and in both directions.

The expected result has been collected using GNU diff 3.10.

lib/diff/lcs/hunk.rb

@@ -331,8 +331,10 @@ def context_range(mode, op)
   def unified_range(mode)
     case mode
     when :old
+      return "0,0" if @old_empty
       s, e = (@start_old + 1), (@end_old + 1)
     when :new
+      return "0,0" if @new_empty
       s, e = (@start_new + 1), (@end_new + 1)
     end
 

spec/fixtures/ldiff/output.diff.empty.vs.four_lines

@@ -0,0 +1,5 @@
+0a1,4
+> one
+> two
+> three
+> four

spec/fixtures/ldiff/output.diff.empty.vs.four_lines-c

@@ -0,0 +1,9 @@
+*** spec/fixtures/empty	2025-01-31 12:14:52.856031635 +0100
+--- spec/fixtures/four_lines	2025-01-31 12:13:45.476036544 +0100
+***************
+*** 0 ****
+--- 1,4 ----
++ one
++ two
++ three
++ four

spec/fixtures/ldiff/output.diff.empty.vs.four_lines-e

@@ -0,0 +1,6 @@
+0a
+one
+two
+three
+four
+.

spec/fixtures/ldiff/output.diff.empty.vs.four_lines-f

@@ -0,0 +1,6 @@
+a0
+one
+two
+three
+four
+.

spec/fixtures/ldiff/output.diff.empty.vs.four_lines-u

@@ -0,0 +1,7 @@
+--- spec/fixtures/empty	2025-01-31 12:14:52.856031635 +0100
++++ spec/fixtures/four_lines	2025-01-31 12:13:45.476036544 +0100
+@@ -0,0 +1,4 @@
++one
++two
++three
++four

spec/fixtures/ldiff/output.diff.four_lines.vs.empty

@@ -0,0 +1,5 @@
+1,4d0
+< one
+< two
+< three
+< four

spec/fixtures/ldiff/output.diff.four_lines.vs.empty-c

@@ -0,0 +1,9 @@
+*** spec/fixtures/four_lines	2025-01-31 12:13:45.476036544 +0100
+--- spec/fixtures/empty	2025-01-31 12:14:52.856031635 +0100
+***************
+*** 1,4 ****
+- one
+- two
+- three
+- four
+--- 0 ----

spec/fixtures/ldiff/output.diff.four_lines.vs.empty-e

@@ -0,0 +1 @@
+1,4d

spec/fixtures/ldiff/output.diff.four_lines.vs.empty-f

@@ -0,0 +1 @@
+d1 4

spec/fixtures/ldiff/output.diff.four_lines.vs.empty-u

@@ -0,0 +1,7 @@
+--- spec/fixtures/four_lines	2025-01-31 12:13:45.476036544 +0100
++++ spec/fixtures/empty	2025-01-31 12:14:52.856031635 +0100
+@@ -1,4 +0,0 @@
+-one
+-two
+-three
+-four

spec/hunk_spec.rb

@@ -72,7 +72,7 @@
 
       it "produces a unified diff" do
         expected = <<-EXPECTED.gsub(/^\s+/, "").encode("UTF-16LE").chomp
-          @@ -1 +1,2 @@
+          @@ -0,0 +1 @@
           +Tu a un carte avec {count} items
         EXPECTED
 

spec/ldiff_spec.rb

@@ -11,6 +11,8 @@
     {:name => "diff.missing_new_line1", :left => "four_lines", :right => "four_lines_with_missing_new_line", :diff => 1},
     {:name => "diff.missing_new_line2", :left => "four_lines_with_missing_new_line", :right => "four_lines", :diff => 1},
     {:name => "diff.issue95_trailing_context", :left => "123_x", :right => "456_x", :diff => 1},
+    {:name => "diff.four_lines.vs.empty", :left => "four_lines", :right => "empty", :diff => 1},
+    {:name => "diff.empty.vs.four_lines", :left => "empty", :right => "four_lines", :diff => 1},
     {:name => "diff.bin1", :left => "file1.bin", :right => "file1.bin", :diff => 0},
     {:name => "diff.bin2", :left => "file1.bin", :right => "file2.bin", :diff => 1},
     {:name => "diff.chef", :left => "old-chef", :right => "new-chef", :diff => 1},