fix(stage): staging of files with no nl at eof

[lewis6991]

Previously when diffing two files where one did not have a newline at the end of the file, gitsigns was unable to stage such differences since this information was not captured during the diff stage.

If you run vim.diff('a', 'a\n') you get the result:

@@ -1 +1 @@
-a
\ No newline at end of file
+a

However if you run vim.diff('a', 'a\n', {result_type='indices'}) you get:

{ {1, 1, 1, 1} }

And since Gitsigns tracks changes as a list of text lines, the information about a missing newline at the end of the file is not correctly tracked.

The main consequence of this is that staging hunks which contain these lines would result in an error as the generated patch would not apply since it was missing "\ No newline at end of file".

To fix this, the internal hunk object now tracks this end of file information and patches are now generated correctly.