t3301: modernize style

Make this test script appear somewhat less old-fashioned:

 - Use test helper functions:
    - write_script
    - test_commit
    - test_write_lines
    - test_line_count
    - test_config
    - test_unconfig
    - test_path_is_missing

 - Remove whitespace between redirection operators and their targets.

 - Move preparation of "expect" files into tests.

 - Rename "output" files to "actual".

 - More consistent quoting, especially around commands that might
   expand to nothing.

 - More visibility of important whitespace with ${indent}.

 - Combine pairs of tests that unnecessarily split setup and verification.

Improved-by: Eric Sunshine <sunshine@sunshineco.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Michael Blume <blume.mike@gmail.com>
Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>