Merge branch 'dt/disable-bitmap-in-auto-gc'

Merge branch 'dt/disable-bitmap-in-auto-gc'

It is natural that "git gc --auto" may not attempt to pack
everything into a single pack, and there is no point in warning
when the user has configured the system to use the pack bitmap,
leading to disabling further "gc".

* dt/disable-bitmap-in-auto-gc:
  repack: die on incremental + write-bitmap-index
  auto gc: don't write bitmaps for incremental repacks