Can I convince the diff algorithm to behave better?

(For a specific, nuanced, and personal definition of better...)

I have a frequent behavior that arises when I am copy/pasting chunks of code, typically in tests. Here is an example:

My Original code:

def function():
   line 1
   line 2
   line 3
   line 4
   line 5
   line 6

-------------------------------- I add, after it:

def function2():
   line 1
   line 2
   line 3
   line 4
   line 5
   line 6

-------------------------------- My diff is:

+   line 3
+   line 4
+   line 5
+   line 6
+
+def function2():
+   line 1
+   line 2

-------------------------------- I'd like my diff to be

+
+def function2():
+   line 1
+   line 2
+   line 3
+   line 4
+   line 5
+   line 6

Obviously there's nothing incorrect about the former diff, I just wish it was the latter rather than the former.

I know that git includes four diff algorithms; in my testing patience or histogram exacerbated the problem; and none of them improved upon it. If anyone has suggestions I'd be curious to know if there's anything that could be done...

Thanks, -tom