t4301: account for behavior differences between sed implementations

It is a common pattern in this script to write the result of
`merge-tree -z` (NUL-termination mode) to an "actual" file and then
manually append a newline to that file so that it can be diff'd easily
with a hand-crafted "expect" file which itself ends with a newline since
it has been created by standard Unix tools which terminate lines by
default. For instance:

    git merge-tree --write-tree -z ... >out &&
    printf "\\n" >>out
    anonymize_hash out >actual &&
    q_to_nul <<-EOF >expect &&
    ...
    EOF
    test_cmp expect actual

However, one test gets this backward:

    git merge-tree --write-tree -z ... >out &&
    anonymize_hash out >actual &&
    printf "\\n" >>actual

which means that, unlike all other cases, when anonymize_hash() is
called, the file being anonymized does not end with a newline. As a
result, this test fails on some platforms.

anonymize_hash() is implemented like this:

    anonymize_hash() {
        sed -e "s/[0-9a-f]\{40,\}/HASH/g" "$@"
    }

The problem arises due to differences in behavior of various `sed`
implementations when fed an incomplete line (lacking a newline).
Although most modern `sed` implementations output such a line
unmolested (i.e. without a newline), some older `sed` implementations
forcibly add a newline to the incomplete line (giving the output an
extra unexpected newline), while other very old implementations simply
swallow an incomplete line and don't emit it at all (making the output
shorter than expected).

Fix this test by manually adding the newline before passing it through
`sed`, thus ensuring identical behavior with all `sed` implementation,
and bringing the test in line with other tests in this script.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4301-merge-tree-write-tree.sh

@@ -760,8 +760,8 @@ test_expect_success 'NUL terminated conflicted file "lines"' '
 	git commit -m "Renamed numbers" &&
 
 	test_expect_code 1 git merge-tree --write-tree -z tweak1 side2 >out &&
+	printf "\\n" >>out &&
 	anonymize_hash out >actual &&
-	printf "\\n" >>actual &&
 
 	# Expected results:
 	#   "greeting" should merge with conflicts