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

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>