perf hists: Link hist entry pairs to leader

perf hists: Link hist entry pairs to leader

Current hists__match/link() link a leader to its pair, so if multiple
pairs were linked, the leader will lose pointer to previous pairs since
it was overwritten.  Fix it by making leader the list head.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1354171126-14387-8-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>