perf diff: Percent calcs should use double values

Otherwise we do integer math and the delta values round up to
multiples of 1.0%.

Also, calculate absolute values. Things look precise now:

$ perf report -i perf.data.old --sort dso,symbol | head -13
     9.02%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     4.88%  find                         [.] 0x00000000014af0
     2.91%  [kernel]                     [k] __kmalloc
     2.85%  [kernel]                     [k] ext4_htree_store_dirent
     2.50%  libc-2.10.1.so               [.] __GI_memmove
     2.44%  [kernel]                     [k] half_md4_transform
     2.43%  [kernel]                     [k] _spin_lock
     2.33%  [kernel]                     [k] system_call
$ perf report -i perf.data --sort dso,symbol | head -13
     8.55%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     3.11%  [kernel]                     [k] __kmalloc
     3.07%  [kernel]                     [k] ext4_htree_store_dirent
     2.66%  find                         [.] 0x00000000016bcf
     2.61%  [kernel]                     [k] _atomic_dec_and_lock
     2.46%  [kernel]                     [k] half_md4_transform
     2.41%  libc-2.10.1.so               [.] __GI_memmove
     2.30%  find                         [.] 0x00000000009219
$ perf diff | head -13
     9.02%     -0.47%  libc-2.10.1.so               [.] _IO_vfprintf_internal
     2.91%     +0.20%  [kernel]                     [k] __kmalloc
     2.85%     +0.23%  [kernel]                     [k] ext4_htree_store_dirent
     1.99%     +0.62%  [kernel]                     [k] _atomic_dec_and_lock
     2.44%     +0.02%  [kernel]                     [k] half_md4_transform
     2.50%     -0.09%  libc-2.10.1.so               [.] __GI_memmove
     1.88%     +0.01%  [kernel]                     [k] __d_lookup
     2.43%     -0.75%  [kernel]                     [k] _spin_lock
     0.97%     +0.62%  [kernel]                     [k] path_get
     1.99%     -0.42%  libc-2.10.1.so               [.] _int_malloc
$

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1260981109-2621-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>