"lib-diff" tests: make "README" and "COPYING" test data smaller

Follow-up the change in 459b8d22e54 (tests: do not borrow from COPYING
and README from the real source, 2015-02-15) by not shipping a full
copy of older versions of the top-level "COPYING" and "README" files.

The tests that use them just need the small blurb at the top of
"COPYING" as test data, or mock data that's dissimilar. Let's provide
that with a "COPYING_test_data" function instead.

We're not replacing this with some other generic test
data (e.g. "lorum ipsum") because these tests require test file header
to be the old "COPYING" file. See e.g. "t4003-diff-rename-1.sh" which
changes the file, and then does full "test_cmp" comparisons on the
resulting "git diff" output.

This change only changes tests that used the "lib-diff.sh" library,
but splits up what they need into a new "lib-diff-data.sh". A
subsequent commit will change related tests that were missed in
459b8d22e54.

For the test in "t4008-diff-break-rewrite.sh" the "README" file can go
away in favor of echoing the line "some dissimilar content" to a file
in the one test that needed it.

The point of that test is to start with files "A" and "B", and then
have A be more similar to the state of "B" than to its old version (by
copying over the content from the "COPYING" file). Just comparing the
pre-image of "some dissimilar content" and later a munged version of
the "COPYING" output serves that purpose.

While we're at it get rid of a stray "echo $tree" debugging line added
in 15d061b435a ([PATCH] Fix the way diffcore-rename records unremoved
source., 2005-05-27), and stop calling "hash-object" to get the hash
of an object we've just added to the index. We can instead extract
that information from the index itself with "rev-parse".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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