The `git merge-recursive` command allows the user to directly specify
three commits to merge -- base, head, and remote.  (More than three can be
specified in the case of multiple merge bases.)  Note that since the user
is allowed to specify head, it need not match HEAD.

Virtually every test and script in the current git.git codebase calls `git
merge-recursive` with head=HEAD, and likely external callers do as well,
which is why this has gone unnoticed.  There is one notable
counter-example: git-stash.sh.  However, git-stash called `git
merge-recursive` with an index that matches the expected merge result,
which happens to be a currently allowed exception to the "index must match
head" rule, so this never triggered an error previously.

Since we would like to tighten up the "index must match head" rule, we
need to make sure we are comparing to the correct head.  Add a testcase
that demonstrates the failure when we check the wrong HEAD.
Signed-off-by: NElijah Newren <newren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>