perf tools: Fix "Command" sort_entry's cmp and collapse function

Currently the se_cmp and se_collapse use pointer comparison,
which is ok for for testing equality of strings. It's not ok
as comparing function for rbtree insertion, because it gives
different results based on current pointer values.

We saw test 32 (hists cumulation test) failing based on different
environment setup. Having all sort functions straightened fix the
test for us.

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>