tests: demonstrate "show --word-diff --color-moved" regression

Add a failing test which demonstrates a regression in
a18d66c ("diff.c: free "buf" in diff_words_flush()", 2022-03-04),
the regression is discussed in detail in the subsequent commit. With
it running `git show --word-diff --color-moved` with SANITIZE=address
would emit:

	==31191==ERROR: AddressSanitizer: attempting double-free on 0x617000021100 in thread T0:
	    #0 0x49f0a2 in free (git+0x49f0a2)
	    #1 0x9b0e4d in diff_words_flush diff.c:2153:3
	    #2 0x9aed5d in fn_out_consume diff.c:2354:3
	    #3 0xe092ab in consume_one xdiff-interface.c:43:9
	    #4 0xe072eb in xdiff_outf xdiff-interface.c:76:10
	    #5 0xec7014 in xdl_emit_diffrec xdiff/xutils.c:53:6
	    [...]

	0x617000021100 is located 0 bytes inside of 768-byte region [0x617000021100,0x617000021400)
	freed by thread T0 here:
	    #0 0x49f0a2 in free (git+0x49f0a2)
	    [...(same stacktrace)...]

	previously allocated by thread T0 here:
	    #0 0x49f603 in __interceptor_realloc (git+0x49f603)
	    #1 0xde4da4 in xrealloc wrapper.c:126:8
	    #2 0x995dc5 in append_emitted_diff_symbol diff.c:794:2
	    #3 0x96c44a in emit_diff_symbol diff.c:1527:3
	    [...]

This was not caught by the test suite because we test `diff
--word-diff --color-moved` only so far.

Therefore, add a test for `show`, too.

Signed-off-by: Michael J Gruber <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: mjg

t/t4015-diff-whitespace.sh

@@ -1622,7 +1622,7 @@ test_expect_success 'cmd option assumes configured colored-moved' '
 	test_cmp expected actual
 '
 
-test_expect_success 'no effect from --color-moved with --word-diff' '
+test_expect_success 'no effect on diff from --color-moved with --word-diff' '
 	cat <<-\EOF >text.txt &&
 	Lorem Ipsum is simply dummy text of the printing and typesetting industry.
 	EOF
@@ -1636,6 +1636,12 @@ test_expect_success 'no effect from --color-moved with --word-diff' '
 	test_cmp expect actual
 '
 
+test_expect_failure 'no effect on show from --color-moved with --word-diff' '
+	git show --color-moved --word-diff >actual &&
+	git show --word-diff >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'set up whitespace tests' '
 	git reset --hard &&
 	# Note that these lines have no leading or trailing whitespace.