merge script: refuse to merge during merge
merge script: refuse to merge during merge

Check MERGE_HEAD and bail out if it exists.  Based on v1.6.3.3~3^2
(refuse to merge during a merge, 2009-06-01).  Without this change,
the scripted merge does not pass t3030.

Cc: Clemens Buchacher <3561697699be488d2983f82b126a1881ffc3afbc@aon.at>
Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git