ref-filter: add ref_format_clear() function

After using the ref-filter API, callers should use ref_filter_clear() to
free any used memory. However, there's not a matching function to clear
the ref_format struct.

Traditionally this did not need to be cleaned up, as it was just a way
for the caller to store and pass format options as a single unit. Even
though the parsing step of some placeholders may allocate data, that's
usually inside their "used_atom" structs, which are part of the
ref_filter itself.

But a few placeholders keep data outside of there. The %(ahead-behind)
and %(is-base) parsers both keep a master list of bases, because they
perform a single filtering pass outside of the use of any particular
atom. And since the format parser does not have access to the ref_filter
struct, they store their cross-atom data in the ref_format struct
itself.

And thus when they are finished, the ref_format also needs to be cleaned
up. So let's add a function to do so, and call it from all of the users
of the ref-filter API.

The %(is-base) case is found by running LSan on t6300. After this patch,
the script can now be marked leak-free.

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

ref-filter.c

@@ -3621,3 +3621,16 @@ void ref_filter_clear(struct ref_filter *filter)
 	free_commit_list(filter->unreachable_from);
 	ref_filter_init(filter);
 }
+
+void ref_format_init(struct ref_format *format)
+{
+	struct ref_format blank = REF_FORMAT_INIT;
+	memcpy(format, &blank, sizeof(blank));
+}
+
+void ref_format_clear(struct ref_format *format)
+{
+	string_list_clear(&format->bases, 0);
+	string_list_clear(&format->is_base_tips, 0);
+	ref_format_init(format);
+}