gdb: make use of scoped_restore in unduplicated_should_be_inserted

gdb: make use of scoped_restore in unduplicated_should_be_inserted

I noticed that we could make use of a scoped_restore in the function
unduplicated_should_be_inserted.  I've also converted the function
return type from int to bool.

This change shouldn't make any difference, as I don't think anything
within should_be_inserted could throw an exception, but the change
doesn't hurt, and will help keep us safe if anything ever changes in
the future.

Approved-By: Simon Marchi <simon.marchi@efficios.com>