t1501 (rev-parse): clarify

t1501 (rev-parse): clarify

Tweak the style of these tests to make them easier to read.

 - Replace test_rev_parse() which produced multiple mini-tests with a
   simple function that can be used with the test body.

 - Combine multiple mini-tests into larger chunks that are easier
   to read.

 - Do not hard-code object IDs.  We may use a different hash some day.

 - Use test_cmp in preference to the test builtin.  The former
   produces useful output when tests are run with the "-v" option.

 - Guard all test code with test_expect_success.  This makes it much
   easier to visually scan through the test and find code of interest.

 - Use subshells to make the current directory easier to track.
   Outside of any subshell, the current directory is always
   $TEST_DIRECTORY now.

Also add a new test demonstrating a possible bug noticed in the
process of cleaning up:  “git rev-parse --show-prefix” leaves out
the trailing newline after an empty prefix when cwd is at the
toplevel of the work tree.

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