[lit] Fix internal diff newlines for -w/-b

For example, without this patch:

```
$ python $LIT_BUILTINS/diff.py -b foo.txt bar.txt
*** /tmp/foo.txt
--- /tmp/bar.txt
***************
*** 1,2 ****
  1! 2--- 1,2 ----
  1! 20
```

With this patch:

```
$ python $LIT_BUILTINS/diff.py -b foo.txt bar.txt
*** /tmp/foo.txt
--- /tmp/bar.txt
***************
*** 1,2 ****
  1
! 2
--- 1,2 ----
  1
! 20
```

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D71577