Elijah Newren authored Dec 21, 2017 and Junio C Hamano committed Dec 22, 2017

builtin/merge.c contains this important requirement for merge strategies:
	/*
	 * At this point, we need a real merge.  No matter what strategy
	 * we use, it would operate on the index, possibly affecting the
	 * working tree, and when resolved cleanly, have the desired
	 * tree in the index -- this means that the index must be in
	 * sync with the head commit.  The strategies are responsible
	 * to ensure this.
	 */

merge-recursive does not do this check directly, instead it relies on
unpack_trees() to do it.  However, merge_trees() has a special check for
the merge branch exactly matching the merge base; when it detects that
situation, it returns early without calling unpack_trees(), because it
knows that the HEAD commit already has the correct result.  Unfortunately,
it didn't check that the index matched HEAD, so after it returned, the
outer logic ended up creating a merge commit that included something
other than HEAD.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>