Fix E539: replace dot-containing highlight group names in gitinfo()

[Copilot]

Neovim's statusline parser rejects . inside %#...# highlight group tokens with E539: Illegal character, killing the render coroutine. gitinfo() was producing strings like %#@diff.plus# and %#@diff.Head# via a prefix + dot-joined alias scheme.

Changes

• lua/modeline/provider.lua
  • Replace alias entries with dot-free names: { 'ModeLineGitHead', 'diffAdded', 'diffChanged', 'diffRemoved' }
  • Drop the @diff. prefix in the format string: %%#@diff.%s# → %%#%s#
  • Define ModeLineGitHead (linked to Title) via nvim_set_hl with default = true at the top of gitinfo()

-- before
local alias = { 'Head', 'plus', 'minus', 'delta' }
('%%#@diff.%s#%s%%*'):format(alias[i], signs[i] .. dict[order[i]])

-- after
vim.api.nvim_set_hl(0, 'ModeLineGitHead', { link = 'Title', default = true })
local alias = { 'ModeLineGitHead', 'diffAdded', 'diffChanged', 'diffRemoved' }
('%%#%s#%s%%*'):format(alias[i], signs[i] .. dict[order[i]])