The following is an easy mistake to make for users coming from version
control systems with an "update and commit"-style workflow.

        1. git pull
        2. resolve conflicts
        3. git pull

Step 3 overrides MERGE_HEAD, starting a new merge with dirty index.
IOW, probably not what the user intended. Instead, refuse to merge
again if a merge is in progress.
Reported-by: NDave Olszewski <cxreg@pobox.com>
Signed-off-by: NClemens Buchacher <drizzd@aon.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>