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 <drizzd@aon.at>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-merge.sh

@@ -27,6 +27,9 @@ cd_to_toplevel
 test -z "$(git ls-files -u)" ||
 	die "Merge is not possible because you have unmerged files."
 
+! test -e "$GIT_DIR/MERGE_HEAD" ||
+	die 'You have not concluded your merge (MERGE_HEAD exists).'
+
 LF='
 '