Make command-repeat work after gdb.execute

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.