perf tools: Make --version show kernel version instead of pull req tag

Before:

  $ perf --version
  perf version perf.urgent.for.mingo.5.g37da28

After:

  $ perf --version
  perf version 3.4.8941.g37da28.dirty

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-vc9b4e6023iegz9kabr3yvyv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/PERF-VERSION-GEN

@@ -12,7 +12,7 @@ LF='
 # First check if there is a .git to get the version from git describe
 # otherwise try to get the version from the kernel makefile
 if test -d ../../.git -o -f ../../.git &&
-	VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
+	VN=$(git describe --match 'v[0-9].[0-9]*' --abbrev=4 HEAD 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;
 	v[0-9]*)