refs/reftable: precompute prefix length

We're recomputing the prefix length on every iteration of the ref
iterator. Precompute it for another speedup when iterating over 1
million refs:

    Benchmark 1: show-ref: single matching ref (revision = HEAD~)
      Time (mean ± σ):     100.3 ms ±   3.7 ms    [User: 97.3 ms, System: 2.8 ms]
      Range (min … max):    97.5 ms … 139.7 ms    1000 runs

    Benchmark 2: show-ref: single matching ref (revision = HEAD)
      Time (mean ± σ):      95.8 ms ±   3.4 ms    [User: 92.9 ms, System: 2.8 ms]
      Range (min … max):    93.0 ms … 121.9 ms    1000 runs

    Summary
      show-ref: single matching ref (revision = HEAD) ran
        1.05 ± 0.05 times faster than show-ref: single matching ref (revision = HEAD~)

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/reftable-backend.c

@@ -346,6 +346,7 @@ struct reftable_ref_iterator {
 	struct object_id oid;
 
 	const char *prefix;
+	size_t prefix_len;
 	unsigned int flags;
 	int err;
 };
@@ -371,8 +372,8 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		if (!starts_with(iter->ref.refname, "refs/"))
 			continue;
 
-		if (iter->prefix &&
-		    strncmp(iter->prefix, iter->ref.refname, strlen(iter->prefix))) {
+		if (iter->prefix_len &&
+		    strncmp(iter->prefix, iter->ref.refname, iter->prefix_len)) {
 			iter->err = 1;
 			break;
 		}
@@ -481,6 +482,7 @@ static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_
 	iter = xcalloc(1, sizeof(*iter));
 	base_ref_iterator_init(&iter->base, &reftable_ref_iterator_vtable, 1);
 	iter->prefix = prefix;
+	iter->prefix_len = prefix ? strlen(prefix) : 0;
 	iter->base.oid = &iter->oid;
 	iter->flags = flags;
 	iter->refs = refs;