Merge branch 'js/empty-index-fixes'

A few places failed to differenciate the case where the index is
truly empty (nothing added) and we haven't yet read from the
on-disk index file, which have been corrected.

* js/empty-index-fixes:
  commit -a -m: allow the top-level tree to become empty again
  split-index: accept that a base index can be empty
  do_read_index(): always mark index as initialized unless erroring out

diff --cc builtin/commit.c
Simple merge

diff --cc read-cache.c
Simple merge