contrib/subtree: convert subtree type check to use case statement

The `subtree_for_commit ()` helper function asserts that the subtree
identified by its parameters are either a commit or tree. This is done
via the `-o` parameter of test, which is discouraged.

Refactor the code to instead use a switch statement over the type.
Despite being aligned with our coding guidelines, the resulting code is
arguably also easier to read.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -641,10 +641,16 @@ subtree_for_commit () {
 	while read mode type tree name
 	do
 		assert test "$name" = "$dir"
-		assert test "$type" = "tree" -o "$type" = "commit"
-		test "$type" = "commit" && continue  # ignore submodules
-		echo $tree
-		break
+
+		case "$type" in
+		commit)
+			continue;; # ignore submodules
+		tree)
+			echo $tree
+			break;;
+		*)
+			die "fatal: tree entry is of type ${type}, expected tree or commit";;
+		esac
 	done || exit $?
 }