ex_global: Catch CTRL-C even if it is mapped.

Problem: If CTRL-C is mapped, it does not interrupt :global output.
Solution: clear `mapped_ctrl_c` during :global.

test/functional/ex_cmds/global_spec.lua

@@ -0,0 +1,74 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+local clear, feed, source = helpers.clear, helpers.feed, helpers.source
+
+if helpers.pending_win32(pending) then return end
+
+describe(':global', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('is interrupted by mapped CTRL-C', function()
+    if os.getenv("TRAVIS") and os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN" then
+      -- XXX: ASAN_UBSAN is too slow to react to the CTRL-C.
+      pending("", function() end)
+      return
+    end
+
+    source([[
+      set nomore
+      set undolevels=-1
+      nnoremap <C-C> <NOP>
+      for i in range(0, 99999)
+        put ='XXX'
+      endfor
+      put ='ZZZ'
+      1
+      .delete
+    ]])
+
+    local screen = Screen.new(52, 6)
+    screen:attach()
+    screen:set_default_attr_ids({
+      [0] = {foreground = Screen.colors.White,
+             background = Screen.colors.Red},
+      [1] = {bold = true,
+             foreground = Screen.colors.SeaGreen}
+    })
+
+    screen:expect([[
+      ^XXX                                                 |
+      XXX                                                 |
+      XXX                                                 |
+      XXX                                                 |
+      XXX                                                 |
+                                                          |
+    ]])
+
+    local function test_ctrl_c(ms)
+      feed(":global/^/p<CR>")
+      helpers.sleep(ms)
+      feed("<C-C>")
+      screen:expect([[
+        XXX                                                 |
+        XXX                                                 |
+        XXX                                                 |
+        XXX                                                 |
+        {0:Interrupted}                                         |
+        Interrupt: {1:Press ENTER or type command to continue}^  |
+      ]])
+    end
+
+    -- The test is time-sensitive. Try with different sleep values.
+    local ms_values = {10, 50, 100}
+    for i, ms in ipairs(ms_values) do
+      if i < #ms_values then
+        local status, _ = pcall(test_ctrl_c, ms)
+        if status then break end
+      else  -- Call the last attempt directly.
+        test_ctrl_c(ms)
+      end
+    end
+  end)
+end)