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

* ew/empty-merge-with-dirty-index-maint:
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