builtin/log: fix leak when showing converted blob contents

In `show_blob_object()`, we proactively call `textconv_object()`. In
case we have a textconv driver for this blob we will end up showing the
converted contents, otherwise we'll show the un-converted contents of it
instead.

When the object has been converted we never free the buffer containing
the converted contents. Fix this to plug this memory leak.

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

t/t4030-diff-textconv.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='diff.*.textconv tests'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 find_diff() {