perf tool: simplify a git command in PERF-VERSION-GEN

Replace a complex git log invocation with a simpler git rev-parse
invocation.  If the command fails, we have larger problems: don't
squelch stderr output.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>