t4207: modernize test

t4207: modernize test

Before adding new tests to t4207-log-decoration-colors.sh, update the
existing test to use modern test conventions. This includes:

1. Use lowercase in test names.

2. Keep all test setup inside the test_expect_success blocks. We need
   to be careful about left whitespace in the broken lines of the input
   file.

3. Do not use 'git' commands on the left side of a pipe.

4. Create a cmp_filtered_decorations helper to perform the 'log', 'sed',
   and test_decode_color manipulations. Move the '--all' option to be an
   argument so we can change that value in future tests.

5. Modify the 'sed' command to use a simpler form that is more
   portable.

The next change will introduce new tests usinge these new conventions.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>