Improve diff support

[bockel]

When using the kanagawa theme with fugitive, the diffs are not properly highlighted per themer diff settings.
While themer does set DiffAdd, DiffChange, DiffDelete, these do not appear to be linked to override the syntax/diff.vim defaults of diffAdded, diffRemoved, diffChanged as included from other plugins such as vim-fugitive.

I've addressed it in my local config with the following, but it should probably be merged into mapper.lua as it appears to apply to all themes:

		use({
			"themercorp/themer.lua",
			config = function()
				require("themer").setup({
					colorscheme = "kanagawa",
					remaps = {
						highlights = {
							globals = {
								base = {
									DiffAdd = {
										fg = "NONE",
									},
									DiffChange = {
										fg = "NONE",
									},
									DiffDelete = {
										fg = "NONE",
									},
									diffAdded = {
										link = "DiffAdd",
									},
									diffRemoved = {
										link = "DiffDelete",
									},
									diffChanged = {
										link = "DiffChange",
									},
								},
							},
						},
					},
				})
			end,
		})

Since syntax/diff.vim sets the fg color and themer sets the bg, I needed to disable the original fg highlights to get a more pleasing highlight.

[bryant-the-coder]

If i am not mistaken, fugitive is not yet supported. :p Use gitsigns :DD