util/PERF-VERSION-GEN is currently executed on every build attempt,
and this script can take a lot of time on trees that are at a
significant git-distance from Linus's tree:

  $ time util/PERF-VERSION-GEN

  real    0m4.343s
  user    0m4.176s
  sys     0m0.140s

It also takes a lot of time if the Git repository is network attached, etc.,
because the commands it uses:

        TAG=$(git describe --abbrev=0 --match "v[0-9].[0-9]*" 2>/dev/null )

has to count commits from the nearest tag and thus has to access (and
decompress) every git commit blob on the relevant version path.

Even on Linus's tree it takes 0.28 seconds on a fast box to count all the
commits and get the git version string:

  $ time util/PERF-VERSION-GEN

  real    0m0.279s
  user    0m0.247s
  sys     0m0.025s

But the version string only has to be regenerated if the git repository's
head commit changes. So add a dependency of ../../.git/HEAD and touch
the file every time it's regenerated, so that Make's build rules can
pick it up and cache the result:

  make: `PERF-VERSION-FILE' is up to date.

  real    0m0.184s
  user    0m0.117s
  sys     0m0.026s

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-wvmlrurufuk6mo1ovtNigguT@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>