t5512: stop losing git exit code in here-docs

t5512: stop losing git exit code in here-docs

The expected references are generated using a here-doc with some inline
command substitutions. If one of the `git rev-parse` invocations within
the command substitutions fails, its return code is swallowed and we
won't know about it. Replace these command substitutions with
generate_references(), which actually reports when `git rev-parse`
fails.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>