Merge branch 'en/merge-trivial-fix'

When "git merge" notices that the merge can be resolved purely at the tree level (without having to merge blobs) and the resulting tree happens to already exist in the object store, it forgot to update the index, which lead to an inconsistent state for later operations. * en/merge-trivial-fix: builtin/merge.c: fix a bug with trivial merges t7605: add a testcase demonstrating a bug with trivial merges