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>

t/t9902-completion.sh

@@ -590,12 +590,10 @@ test_expect_success '__gitcomp - doesnt fail because of invalid variable name' '
 	__gitcomp "$invalid_variable_name"
 '
 
-read -r -d "" refs <<-\EOF
-main
+refs='main
 maint
 next
-seen
-EOF
+seen'
 
 test_expect_success '__gitcomp_nl - trailing space' '
 	test_gitcomp_nl "m" "$refs" <<-EOF