fix(runtime): source c ftplugin properly for cpp on Windows (#29053)

On Windows, '{' is currently not treated as a wildcard char, so another
wildcard char is needed for the pattern to be treated as a wildcard.

It may be worth trying to make '{' always a wildcard char in the future,
but that'll be a bit harder as it'll be necessary to make sure '{' is
escaped at various places.

runtime/ftplugin/cpp.vim

@@ -10,7 +10,8 @@ if exists("b:did_ftplugin")
 endif
 
 " Behaves mostly just like C
-runtime! ftplugin/c.{vim,lua} ftplugin/c_*.{vim,lua} ftplugin/c/*.{vim,lua}
+" XXX: "[.]" in the first pattern makes it a wildcard on Windows
+runtime! ftplugin/c[.]{vim,lua} ftplugin/c_*.{vim,lua} ftplugin/c/*.{vim,lua}
 
 " C++ uses templates with <things>
 " Disabled, because it gives an error for typing an unmatched ">".

test/functional/lua/runtime_spec.lua

@@ -401,4 +401,12 @@ describe('runtime:', function()
       eq('ABab', eval('g:seq'))
     end)
   end)
+
+  it('cpp ftplugin loads c ftplugin #29053', function()
+    eq('', eval('&commentstring'))
+    eq('', eval('&omnifunc'))
+    exec('edit file.cpp')
+    eq('/*%s*/', eval('&commentstring'))
+    eq('ccomplete#Complete', eval('&omnifunc'))
+  end)
 end)