test-lib: let test_merge() perform octopus merges

Currently test_merge() only allows developers to merge in one branch.
However, this restriction is artificial and there is no reason why it
needs to be this way.

Extend test_merge() to allow the specification of multiple branches so
that octopus merges can be performed.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -228,9 +228,11 @@ test_commit () {
 # can be a tag pointing to the commit-to-merge.
 
 test_merge () {
+	label="$1" &&
+	shift &&
 	test_tick &&
-	git merge -m "$1" "$2" &&
-	git tag "$1"
+	git merge -m "$label" "$@" &&
+	git tag "$label"
 }
 
 # Efficiently create <nr> commits, each with a unique number (from 1 to <nr>