Make command-repeat work after gdb.execute
    
    Since commit
    
      56bcdbea2bed ("Let gdb.execute handle multi-line commands")
    
    command repetition after using the `gdb.execute` Python function
    fails (the previous command is not repeated anymore). This happens
    because read_command_lines_1 sets dont_repeat, but the call to
    prevent_dont_repeat in execute_gdb_command is later.
    
    The fix is to move the call to prevent_dont_repeat to the beginning of
    the function.
    
    Tested on my laptop (ArchLinux-x86_64).
    
    gdb/ChangeLog:
    
    	PR python/23714
    	* gdb/python/python.c (execute_gdb_command): Call
    	prevent_dont_repeat earlier to avoid affecting dont_repeat.
    
    gdb/testuite/ChangeLog:
    
    	PR python/23714
    	* gdb.python/python.exp: Test command repetition after
    	gdb.execute.