Don't enqueue a repaint in the middle of one

This can easily lead to an infinite loop, if a variable handler
triggers a repaint and the variable is set in the prompt, e.g. some of
the git variables.

A simple way to reproduce:

    function fish_mode_prompt
        commandline -f repaint
    end

Repainting executes the mode prompt, which triggers a repaint, which
triggers the mode prompt, ....

So we just set a flag and check it.

Fixes #7324.

src/parser.h

@@ -151,6 +151,10 @@ struct library_data_t {
     /// Number of recursive calls to builtin_complete().
     uint32_t builtin_complete_recursion_level{0};
 
+    /// If we're currently repainting the commandline.
+    /// Useful to stop infinite loops.
+    bool is_repaint{false};
+
     /// Whether we called builtin_complete -C without parameter.
     bool builtin_complete_current_commandline{false};