perf hists: Introduce hists__link

That given two hists will find the hist_entries (buckets) in the second
hists that are for the same bucket in the first and link them, then it
will look for all buckets in the second that don't have a counterpart in
the first and will create a dummy counterpart that will then be linked
to the entry in the second.

For multiple events this will be done pairing the leader with all the
other events in the group, so that in the end the leader will have all
the buckets in all the hists in a group, dummy or not while the other
hists will be left untouched.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-l9l9ieozqdhn9lieokd95okw@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>