reftable/record: reuse refnames when decoding log records

When decoding a log record we always reallocate their refname arrays.
This results in quite a lot of needless allocation churn.

Refactor the code to grow the array as required only. Like this, we
should usually only end up reallocating the array a small handful of
times when iterating over many refs. Before:

    HEAP SUMMARY:
        in use at exit: 13,473 bytes in 122 blocks
      total heap usage: 4,068,487 allocs, 4,068,365 frees, 332,011,793 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 13,473 bytes in 122 blocks
      total heap usage: 3,068,488 allocs, 3,068,366 frees, 307,122,961 bytes allocated

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

reftable/reftable-record.h

@@ -74,6 +74,7 @@ int reftable_ref_record_equal(const struct reftable_ref_record *a,
 /* reftable_log_record holds a reflog entry */
 struct reftable_log_record {
 	char *refname;
+	size_t refname_cap;
 	uint64_t update_index; /* logical timestamp of a transactional update.
 				*/