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. There is a gotcha though: when the delimiter
isn't found at all, then the read builtin will return an error. 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 silencing the error.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

t/t9902-completion.sh

@@ -590,7 +590,7 @@ test_expect_success '__gitcomp - doesnt fail because of invalid variable name' '
 	__gitcomp "$invalid_variable_name"
 '
 
-read -r -d "" refs <<-\EOF
+read -r -d "" refs <<-\EOF || :
 main
 maint
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