Merge branch 'ew/empty-merge-with-dirty-index'

"git merge -s recursive" did not correctly abort when the index is
dirty, if the merged tree happened to be the same as the current
HEAD, which has been fixed.

* ew/empty-merge-with-dirty-index:
  merge-recursive: avoid incorporating uncommitted changes in a merge
  move index_has_changes() from builtin/am.c to merge.c for reuse
  t6044: recursive can silently incorporate dirty changes in a merge

diff --cc builtin/am.c
Simple merge