perf diff: Switching the base hists to be pairs head

perf diff: Switching the base hists to be pairs head

Making the baseline hists to act as a pairs head.

So far we don't care which hists act as a pairs head, because we have
only 2 files to deal with and any of them is suitable to do the job.

But if we want to process more files, we need to pick up one hists to
act as pairs head, and the baseline hists is the most suitable.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-cklmt2o4j87i9viz900245ae@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>