Fix diff highlight tweaks

Currently the HunkDiffAddAsDelete expects the `bg` property of the
DiffDelete highlight group to be present which isn't true for all
colorschemes.

This updates the highlight group mapping to take all properties from the
DiffDelete hl group and map them to hex with proper presence checks.

Fixes #8

lua/hunk/api/highlights.lua

@@ -6,15 +6,28 @@ function M.set_win_hl(winid, highlights)
   })
 end
 
+local function color_as_hex(color)
+  if not color then
+    return
+  end
+  return string.format("#%06x", color)
+end
+
 function M.define_highlights()
   local diff_delete_highlight = vim.api.nvim_get_hl(0, {
     name = "DiffDelete",
     link = true,
   })
 
-  vim.api.nvim_set_hl(0, "HunkDiffAddAsDelete", {
-    bg = string.format("#%06x", diff_delete_highlight.bg),
-  })
+  vim.api.nvim_set_hl(
+    0,
+    "HunkDiffAddAsDelete",
+    vim.tbl_deep_extend("force", diff_delete_highlight, {
+      bg = color_as_hex(diff_delete_highlight.bg),
+      fg = color_as_hex(diff_delete_highlight.fg),
+      sp = color_as_hex(diff_delete_highlight.sp),
+    })
+  )
 
   vim.api.nvim_set_hl(0, "HunkDiffDeleteDim", {
     default = true,