type_from_string_gently: make sure length matches

type_from_string_gently: make sure length matches

When commit fe8e3b7 refactored type_from_string to allow
input that was not NUL-terminated, it switched to using
strncmp instead of strcmp. But this means we check only the
first "len" bytes of the strings, and ignore any remaining
bytes in the object_type_string. We should make sure that it
is also "len" bytes, or else we would accept "comm" as
"commit", and so forth.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>