Fix undo hunk
Fix undo hunk

Fixes #345

When `new_count` is `0` it's a special case meaning that lines should be inserted after `new_line`, not instead of it.

When you remove `line 2` from file:
```
line 1
line 2
line 3
line 4
```
the diff looks line this:
```diff
index 9c2a709..cbf7f40 100644
--- a/test.txt
+++ b/test.txt
@@ -2 +1,0 @@ line 1
-line 2
```

It means `line 2` in `undo` should be inserted below line 1 (`new_line`) and before it was inserted after line 0 (`new_line - 1`).

mhinz/vim-signify