tests/test-fastannotate-diffopts.t

py3: replace `time.clock()` with `time.perf_counter()`

The former was removed in 3.8.  The util function uses `time.clock()` if the
latter is unavailable.

Differential Revision: https://phab.mercurial-scm.org/D7779

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > fastannotate=
  > EOF

  $ hg init repo
  $ cd repo

changes to whitespaces

  $ cat >> a << EOF
  > 1
  > 
  >  
  >  2
  > EOF
  $ hg commit -qAm '1'
  $ cat > a << EOF
  >  1
  > 
  > 2
  > 
  > 
  > 3
  > EOF
  $ hg commit -m 2
  $ hg fastannotate -wB a
  0:  1
  0: 
  1: 2
  0: 
  1: 
  1: 3