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