add linediff extension (#2757)

autoload/airline/extensions/linediff.vim

@@ -0,0 +1,14 @@
+" MIT License. Copyright (c) 2026 Shad
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+function! airline#extensions#linediff#init(ext)
+  call a:ext.add_statusline_func('airline#extensions#linediff#apply')
+endfunction
+
+function! airline#extensions#linediff#apply(...)
+  if exists('b:differ.description')
+    let w:airline_section_c = b:differ.description
+  endif
+endfunction