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

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 <redacted>
Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>