perf_counter tools: Shorten the DSO names using cwd

[acme@emilia linux-2.6-tip]$ pwd
/home/acme/git/linux-2.6-tip

Before (still available using -P/--full-paths)

[acme@emilia linux-2.6-tip]$ perf report -P | head -10
    11.48%             perf: 7454 [kernel]: clear_page_c
     4.89%             perf: 7454 [kernel]: vsnprintf
     4.61%             perf: 7454 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__find_symbol
     4.09%             perf: 7454 [kernel]: number
     4.06%             perf: 7454 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: dso__fprintf
     4.00%             perf: 7454 /home/acme/git/linux-2.6-tip/Documentation/perf_counter/perf: symbol_filter

New default:

[acme@emilia linux-2.6-tip]$ perf report | head -10
    11.48%             perf: 7454 [kernel]: clear_page_c
     4.89%             perf: 7454 [kernel]: vsnprintf
     4.61%             perf: 7454 ./Documentation/perf_counter/perf: dso__find_symbol
     4.09%             perf: 7454 [kernel]: number
     4.06%             perf: 7454 ./Documentation/perf_counter/perf: dso__fprintf
     4.00%             perf: 7454 ./Documentation/perf_counter/perf: symbol_filter

Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20090529164859.GN4747@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>