reftable/block: reuse buffer to compute record keys

When iterating over entries in the block iterator we compute the key of
each of the entries and write it into a buffer. We do not reuse the
buffer though and thus re-allocate it on every iteration, which is
wasteful.

Refactor the code to reuse the buffer.

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

reftable/block.h

@@ -84,10 +84,12 @@ struct block_iter {
 
 	/* key for last entry we read. */
 	struct strbuf last_key;
+	struct strbuf key;
 };
 
 #define BLOCK_ITER_INIT { \
 	.last_key = STRBUF_INIT, \
+	.key = STRBUF_INIT, \
 }
 
 /* initializes a block reader. */