tests/test-merge2.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.

  $ hg init t
  $ cd t
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ rm b
  $ hg update 0
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo This is file b2 > b
  $ hg add b
  $ hg commit -m "commit #2"
  created new head
  $ cd ..; rm -r t

  $ mkdir t
  $ cd t
  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ rm b
  $ hg update 0
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo This is file b2 > b
  $ hg commit -A -m "commit #2"
  adding b
  created new head
  $ cd ..; rm -r t

  $ hg init t
  $ cd t
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ rm b
  $ hg remove b
  $ hg update 0
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo This is file b2 > b
  $ hg commit -A -m "commit #2"
  adding b
  created new head

  $ cd ..