perf diff: Start moving to support matching more than two hists

perf diff: Start moving to support matching more than two hists

We want to match more than two hists, so that we can match more than two
perf.data files and moreover, match hist_entries (buckets) in multiple
events in a group.

So the "baseline"/"leader" will instead of a ->pair pointer, use a
list_head, that will link to the pairs and hists__match use it.

Following that perf_evlist__link will link the hists in its evsel
groups.

Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>