perf tools: Fix perf version generation

perf tools: Fix perf version generation

The tag of the perf version is wrongly determined, always the latest tag
is taken regardless of the HEAD commit:

 $ perf --version
 perf version 3.9.rc8.gd7f5d3
 $ git describe d7f5d3
 v3.9-rc7-154-gd7f5d33
 $ head -n 4 Makefile
 VERSION = 3
 PATCHLEVEL = 9
 SUBLEVEL = 0
 EXTRAVERSION = -rc7

In other cases no tag might be found.

This patch fixes this.

This new implementation handles also the case if there are no tags at
all found in the git repo but there is a commit id.

Signed-off-by: Robert Richter <robert.richter@calxeda.com>
Link: http://lkml.kernel.org/r/1368006214-12912-1-git-send-email-rric@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>