t0000: modernise style

Match the style to more modern test scripts, namely:

 - Prefer tabs for indentation.

 - The first line of each test has prereq, title and opening sq for the
   script body.

 - Move cleanup or initialization of data used by a test inside the test
   itself.

 - Put a newline before the closing sq for each test.

 - Don't conclude the test descriptions with a full stop.

 - Prefer 'test_line_count = COUNT FILE' over 'test $(wc -l <FILE) = COUNT'

 - Prefer 'test_line_count = 0 FILE' over 'cmp -s /dev/null FILE'

 - Use '<<-EOF' style for here documents, so that they can be indented
   as well.  Bot don't do that in case the resulting lines would be too
   long.  Also when there is no $variable_substitution in the body of a
   here document, quote \EOF.

 - Don't redirect the output of commands to /dev/null unconditionally,
   the git testing framework should already take care of handling test
   verbosity transparently and uniformly.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>