t5521: fix and modernize

t5521: fix and modernize

All of these tests were bogus, as they created new directory and tried to
run "git pull" without even running "git init" in there.  They were mucking
with the repository in $TEST_DIRECTORY.

While fixing it, modernize the style not to chdir around outside of
subshell.  Otherwise a failed test will take us to an unexpected directory
and we need to chdir back to the test directory in each test, which is
ugly and error prone.

Signed-off-by: Junio C Hamano <gitster@pobox.com>