Problem: colorschemes with `transparent_mode = true` set Normal
with no bg. `compute_highlight_groups` fell back to `0x1a1a1a`
and blended all highlight groups against it, producing wrong
colors. The `ColorScheme` autocmd also failed to overwrite stale
values because every `nvim_set_hl` call used `default = true`.
Solution: detect transparent Normal via `not normal.bg`. When
transparent, pass `bg = normal.bg` (nil) to `DiffsClear` and
use colorscheme DiffAdd/DiffDelete colors directly without
blending. Reset `hl_retry_pending` on `ColorScheme` so the
retry mechanism re-arms after a theme switch.
Barrett Ruth
6d38472b39
