tests/test-fastannotate-diffopts.t

convert: don't drop commits that are empty in the source when using --filemap

I ran into this when using `hg lfconvert --to-normal` (which uses the filemap
class internally), and saw that commits with nothing but a branch change were
dropped.  We could put in an option that only lfconvert uses internally.  But
silently dropping anything other than a commit where all changes were excluded
seems unintended.  For example, there's a message in mercurial_sink.putcommit()
if it drops an empty commit.  (And the reason that isn't kicking in here is
because lfconvert isn't passing --filemap, so the self.filemapmode conditional
there is always False.)

The naive change of `return not files` broke test-convert-filemap.t, so this is
a little more elaborate than needed for converting from largefiles.

  $ 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