refuse to merge during a merge
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: Dave Olszewski <cxreg@pobox.com>
Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>