line-log: always allocate the output prefix

line-log: always allocate the output prefix

The returned string by `output_prefix()` is sometimes a string constant
and sometimes an allocated string. This has been fine until now because
we always leak the allocated strings, and thus we never tried to free
the string constant.

Fix the code to always return an allocated string and free the returned
value at all callsites.

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