t9902: fix use of `read` with `set -e`

In t9902 we're using the `read` builtin to read some values into a variable. This is done by using `-d ""`, which cause us to read until the end of the heredoc. As the read is terminated by EOF, the command will end up returning a non-zero error code. This hasn't been an issue until now as we didn't run with `set -e`, but that'll change in a subsequent commit. Prepare for this change by not using read at all, as we can simply store the multi-line value directly. Suggested-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>