[Danthewaann]

Describe what this PR does / why we need it

Adds new OctoReviewDiffDelete and OctoReviewDiffAdd highlights so users can better customise the colours that are used in the diff view. This follows up on #1500. These new highlights default to DiffDelete and DiffAdd so nothing changes for existing users.

Does this pull request fix one issue?

NONE

Describe how to verify it

Adjust the OctoReviewDiffDelete OctoReviewDiffAdd highlights via vim.api.nvim_set_hl e.g. vim.api.nvim_set_hl(0, "OctoReviewDiffDelete", {bg = "#172a3a"}). Then open up a review with some changes and you should see the deleted lines now have #172a3a as their background colour.

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt

[disrupted]

I would very much appreciate this getting merged as it would enable the use of custom colorschemes.

the current way is a non-starter. for reference, the complicated workaround that's currently necessary

local constants = require 'octo.constants'
for _, ns in ipairs {
    constants.OCTO_REVIEW_LEFT_HIGHLIGHT_NS,
    constants.OCTO_REVIEW_RIGHT_HIGHLIGHT_NS,
} do
    vim.api.nvim_set_hl(ns, 'DiffAdd', { bg = '#abc' }) -- custom color
    vim.api.nvim_set_hl(ns, 'DiffDelete', { bg = '#eee' })
end

[williambdean]

Thanks @Danthewaann !