Fix diff parser so it tolerates to diff special markers in the content

lib/gitlab/diff/parser.rb

@@ -17,7 +17,9 @@ module Gitlab
         # without having to instantiate all the others that come after it.
         Enumerator.new do |yielder|
           @lines.each do |line|
-            next if filename?(line)
+            # We're expecting a filename parameter only in a meta-part of the diff content
+            # when type is defined then we're already in a content-part
+            next if filename?(line) && type.nil?
 
             full_line = line.delete("\n")
 

spec/lib/gitlab/diff/parser_spec.rb

@@ -143,4 +143,21 @@ eos
     it { expect(parser.parse([])).to eq([]) }
     it { expect(parser.parse(nil)).to eq([]) }
   end
+
+  describe 'tolerates special diff markers in a content' do
+    it "counts lines correctly" do
+      diff = <<~END
+        --- a/test
+        +++ b/test
+        @@ -1,2 +1,2 @@
+        +ipsum
+        +++ b
+        -ipsum
+      END
+
+      lines = parser.parse(diff.lines).to_a
+
+      expect(lines.size).to eq(3)
+    end
+  end
 end