perf tools: Further speed up the perf build

There's another source of overhead in the perf version string generator:

   git update-index -q --refresh

... which will iterate the whole checked out tree. This can be pretty
slow on NFS volumes, but takes some time even with local SSD disks and a
fully cached kernel tree:

 $ perf stat --null --repeat 3 --pre "rm -f PERF-VERSION-FILE" util/PERF-VERSION-GEN
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty

 Performance counter stats for 'util/PERF-VERSION-GEN' (3 runs):

       0.306999221 seconds time elapsed                                          ( +-  0.56% )

So remove the .dirty differentiator as well - it adds little information
because locally patched git trees are common, but seldom are the perf
tools modified.

So a lot of version strings are reported as 'dirty' while in fact they
are pristine perf builds. For example 99% of my perf builds are not
patched but the kernel tree is slightly patched, which adds the .dirty
tag.

Eliminating that tag speeds up version generation by another order of
magnitude:

 $ perf stat --null --repeat 3 --sync --pre "rm -f PERF-VERSION-FILE" util/PERF-VERSION-GEN
 PERF_VERSION = 3.7.rc3.g4b0bd3
 PERF_VERSION = 3.7.rc3.g4b0bd3
 PERF_VERSION = 3.7.rc3.g4b0bd3

 Performance counter stats for 'util/PERF-VERSION-GEN' (3 runs):

       0.021270923 seconds time elapsed                                          ( +-  1.94% )

(Also clean up some of the comments around this code.)

Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andrew Vagin <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Howells <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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