Fix the only incorrect case found by command_structure_invariants selftest.

Fix the only incorrect case found by command_structure_invariants selftest.

The next commit introduces a selftest that detects when the GDB
command structure does not define a tree when using the pointers
'next/*prefixlist'.  This test detects one such case, fixed
by this commit.

The command 'info set' was defined as a specific prefix command,
but re-using the command list already used for the 'show' command.
This leads to the command tree 'next/*prefixlist' to not be a tree.

This change defines 'info set ' as an alias, thereby fixing the selftest.

2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-cmds.c (_initialize_cli_cmds): Define 'info set' as
	an alias of 'show'.