🐛 `syntax` not respected for `*-non-emph-style`

Hi! I'm noticing that if I have the following style settings:

```
minus-style = red
minus-emph-style = bold underline red
minus-non-emph-style = syntax
plus-style = green
plus-emph-style = bold underline green
plus-non-emph-style = syntax
```

Then the `minus/plus-non-emph-style` seems to use `normal` instead of `syntax` highlighting:

However, if I use:

```
minus-style = syntax
minus-emph-style = bold underline red
minus-non-emph-style = syntax
plus-style = syntax
plus-emph-style = bold underline green
plus-non-emph-style = syntax
```

Then the `minus/plus-non-emph-style` will use `syntax` as expected:

However, I'm trying to get it such that only the meaningful changes are red/green while everything else is syntax highlighted. I could see why this would be difficult to implement, so I wasn't sure if this was expected behavior. Couldn't find it documented anywhere.

Fantastic plugin btw, I really appreciate all the work done on this 🙂

If it helps, the raw output of the screenshotted section using `git --no-pager diff`:
```
" buffer navigation
nnoremap
nnoremap l :Buffers
-nnoremap { :bprevious
-nnoremap } :bnext
-nnoremap Q :bprevious bdelete #
+nnoremap [ :bprevious
+nnoremap ] :bnext
nnoremap ; :BLines
nnoremap : :Lines
+nnoremap Q :bprevious bdelete #

" tab navigation
-nnoremap [ :tabp
-nnoremap ] :tabn
+nnoremap { :tabp
+nnoremap } :tabn
```

Reproduce the issue with the reported minus/plus style settings and a git diff containing changed and unchanged lines, then compare the rendering with the configuration where the main styles use syntax. Done means non-emphasized unchanged content retains syntax highlighting while meaningful changes remain red or green.

Written by the indexing model from the issue text.