t7501 (commit): modernize style

t7501 (commit): modernize style

Put the opening quote starting each test on the same line as the
test_expect_* invocation.  While at it:

- guard commands that prepare test input for individual tests in
  the same test_expect_success, so their scope is clearer and
  errors at that stage can be caught;
- use the compare_diff_patch helper function when comparing patches;
- use single-quotes in preference to double-quotes and <<\EOF in
  preference to <<EOF, to save readers the trouble of looking for
  variable interpolations;
- lift the setting of the $author variable used throughout the
  test script to the top of the test script;
- include "setup" in the titles of test assertions that prepare for
  later ones to make it more obvious which tests can be skipped;
- use test_must_fail instead of "if ...; then:; else false; fi",
  for clarity and to catch segfaults when they happen;
- break up some pipelines into separate commands that read and write
  to ordinary files, and test the exit status at each stage;
- chain commands with &&.  Breaks in a test assertion's && chain can
  potentially hide failures from earlier commands in the chain;
- combine two initial tests that do not make as much sense alone.

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