builtin/blame: fix leaking blame entries with --incremental

When passing `--incremental` to git-blame(1) we exit early by jumping to
the `cleanup` label. But some of the cleanups we perform are handled
between the `goto` and its label, and thus we leak the data.

Move the cleanups after the `cleanup` label. While at it, move the logic
to free the scoreboard's `final_buf` into `cleanup_scoreboard()` and
drop its `const` declaration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t8005-blame-i18n.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='git blame encoding conversion'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 if ! test_have_prereq ICONV