ref-filter: fix leak with %(describe) arguments

ref-filter: fix leak with %(describe) arguments

When we parse a %(describe) placeholder, we stuff its arguments into a
strvec, which is then detached into the used_atom struct. But later,
when ref_array_clear() frees the atom, we never free the memory.

To solve this, we just need to add the appropriate free() calls. But
it's a little awkward, since we have to free each element of the array,
in addition to the array itself. Instead, let's store the actual strvec,
which lets us do a simple strvec_clear().

This clears up one case that LSan finds in t6300, but there are more.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>