t1403: split 'show-ref --exists' tests into a separate file

t1403: split 'show-ref --exists' tests into a separate file

The test file for git-show-ref(1), `t1403-show-ref.sh`, contains a group
of tests for the '--exists' flag. To improve organization and to prepare
for refactoring these tests to be shareable, move the '--exists' tests
and their corresponding setup logic into a self-contained test suite,
`t1422-show-ref-exists.sh`.

This is a pure code-movement refactoring with no change in test coverage
or behavior.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>