t4200 (rerere): modernize style

Guard all test code with test_expect_success to make the
script easier to follow.  While at it, pick some other nits:

 - use test_tick (more than we have to, to be realistic);

 - 'single quotes' and \escaped HERE documents where possible
   simplify review for escaping problems;

 - omit whitespace after >redirection operators for
   consistency with other tests;

 - use "update-index --refresh" instead of testing that
   "ls-files -u" output is empty, since the former produces
   nicer output on failure;

 - compare to expected nonempty "ls-files -u" output instead
   of counting lines when it is expected to be nonempty.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>