Don't walk diff trees indefinitely when applying suppressions

Don't walk diff trees indefinitely when applying suppressions

When applying suppressions to diff graphs, if the same diff node
appears twice in the graph in a way that creates a cycle, we can get
trapped in the cycle when walking the graph.  That results in a an
infinite loop.  Note that the infinite loop appears in the
diff::traverse member function, in the for-loop that walks the
children nodes of a given node.

This patch avoids walking the same node twice when applying
suppressions.

The test binaries that exhibit the issue come from the two following
packages:

rh-mariadb101-mariadb-10.1.14-2.el6 and	rh-mariadb101-mariadb-10.1.16-1.el6.

The sub-packages compared are
rh-mariadb101-mariadb-server-10.1.14-2.el6.x86_64.rpm and
rh-mariadb101-mariadb-server-10.1.16-1.el6.x86_64.rpm.

The debug info packages are
rh-mariadb101-mariadb-debuginfo-10.1.16-1.el6.x86_64.rpm and
rh-mariadb101-mariadb-debuginfo-10.1.14-2.el6.x86_64.rpm.

Once the packages are properly extracted the command line that
exhibits the infinite loop is:

    abidiff --d1 rh-mariadb101-mariadb-10.1.14-2.el6.x86_64/usr/lib/debug --d2 rh-mariadb101-mariadb-10.1.16-1.el6.x86_64/usr/lib/debug rh-mariadb101-mariadb-10.1.14-2.el6.x86_64/opt/rh/rh-mariadb101/root/usr/lib64/mysql/plugin/ha_connect.so rh-mariadb101-mariadb-10.1.16-1.el6.x86_64/opt/rh/rh-mariadb101/root/usr/lib64/mysql/plugin/ha_connect.so

This patch has no test because of the cheer size of these packages;
the debug info packages alone take more than 100MB :-( I really hope
we set up a separate repository with big test packages to overcome
this.  Maybe something using fedabipkgdiff ...

Signed-off-by: Dodji Seketeli <dodji@redhat.com>