merge script: forbid merge -s index

Some git-merge-* commands are not merge strategies.  This is based on
v1.6.1-rc1~294^2~7 (builtin-merge: allow using a custom strategy,
2008-07-30) but it is less smart: we just use a hard-coded list of
forbidden strategy names.  It is okay if this falls out of date, since
the code is just an example.

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

contrib/examples/git-merge.sh

@@ -36,6 +36,7 @@ LF='
 
 all_strategies='recur recursive octopus resolve stupid ours subtree'
 all_strategies="$all_strategies recursive-ours recursive-theirs"
+not_strategies='base file index tree'
 default_twohead_strategies='recursive'
 default_octopus_strategies='octopus'
 no_fast_forward_strategies='subtree ours'
@@ -196,6 +197,10 @@ parse_config () {
 				use_strategies="$use_strategies$1 "
 				;;
 			*)
+				case " $not_strategies " in
+				*" $1 "*)
+					false
+				esac &&
 				type "git-merge-$1" >/dev/null 2>&1 ||
 					die "available strategies are: $all_strategies"
 				use_strategies="$use_strategies$1 "