t1302 (core.repositoryversion): style tweaks

t1302 (core.repositoryversion): style tweaks

This test is from 2007, which is late enough for the style to be
recognizably modern but still a while ago.  Freshen it up to
follow new best practices:

 - guard setup commands with test_expect_setup, so errors at
   that stage can be caught;
 - use <<\EOF in preference to <<EOF, to save reviewers the
   trouble of looking for variable interpolations;
 - use test_cmp instead of test "$foo" = "$bar", for better
   output with -v on failure;
 - indent commands in subshells and let them span multiple lines;
 - combine the two "gitdir required mode" tests that do not make
   as much sense alone.

Cc: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>