nvim: statusline: Make winbar status be relative to current directory

We want file path being shown in winbar to be relative to the current
directory or in our case frequently the git root. For whatever reason,
only the first opened leftmost split would show the file path relative
to current git root while any files opened later would show the full
path starting from `HOME`/~ directory. So something around how '%f%m%r'
actually works or how it is being used to set winbar has been a problem.

nvim/.config/nvim/after/ftplugin/c.lua

@@ -7,3 +7,10 @@ vim.bo.makeprg = 'meson compile -C build'
 
 vim.keymap.set('n', 'gh', ":ClangdSwitchSourceHeader<CR>", { noremap=true, buffer=0 })
 vim.keymap.set('n', 'gq', ":!gst-indent %<CR>"           , { noremap=true, buffer=0 })
+
+vim.api.nvim_create_autocmd({ "BufReadPre", "BufWinEnter" }, {
+  pattern  = { "*/usr/include/*" },
+  callback = function()
+    vim.bo.buflisted = false
+  end,
+})

nvim/.config/nvim/after/plugin/statusline.lua

@@ -294,7 +294,10 @@ api.nvim_create_autocmd({"BufWinEnter", "BufFilePost"}, {
       return
     end
 
-    vim.opt_local.winbar = string.format(" %s%s", "%#WinbarFile#", '%f%m%r')
+    -- `:.` Reduces file name to be relative to current directory, if possible.
+    local winbar_file = vim.fn.expand('%:.')
+
+    vim.opt_local.winbar = string.format(" %s%s", "%#WinbarFile#", winbar_file)
   end,
 })
 api.nvim_create_autocmd({"TermOpen", "TermEnter"}, {