unlet g:loaded_clipboard_provider on clipboard reload

With the upstream merge of

    neovim/neovim#10161

we can now safely reload the clipboard provider. There is no tagged
version of neovim with this yet but the only difference is the unlet
command which can be made to ignore errors if we are still running older
neovim version (i.e. <= 0.3.8).

Author: equalsraf

src/gui/runtime/plugin/nvim_gui_shim.vim

@@ -165,14 +165,16 @@ function GuiClipboard()
           \   },
           \ }
 
-	" When the clipboard provider is sourced it short circuits if it cannot
-	" find a working clipboard - this behaviour is used internally by nvim to
-	" check if the provider is available, by checking for
-	" provider#clipboard#Call which is not defined if no clipboard is
-	" available.
+	" We need to reload the neovim clipboard provider here so it picks up on
+	" g:clipboard. In older versions of neovim (<=0.3.8) the provider would
+	" short circuit if a working clipboard was not available. After 0.3.8
+	" the provider should not short circuit, andunsetting
+	" g:loaded_clipboard_provider will enable a full reload of the provider.
 	"
-	" TLDR; we need to source this to reinitialize the clipboard provider
-	runtime autoload/provider/clipboard.vim
+	" TLDR; source this to reinitialize the clipboard provider, this may not
+	" work
+    unlet! g:loaded_clipboard_provider
+    runtime autoload/provider/clipboard.vim
 endfunction
 
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