Use pool for loose refdb string allocations

Instead of using lots of strdup calls, this adds a memory pool to
the loose refs iteration code and uses it for keeping track of the
loose refs array.  Memory usage could probably be reduced even
further by eliminating the vector and just scanning by adding the
strlen of each ref, but that would be a more intrusive changes.

This also updates the error handling to be more thorough about
checking for failed allocations, etc.

src/refdb_fs.c

@@ -560,7 +560,10 @@ typedef struct {
 	git_reference_iterator parent;
 
 	char *glob;
+
+	git_pool pool;
 	git_vector loose;
+
 	unsigned int loose_pos;
 	khiter_t packed_pos;
 } refdb_fs_iter;
@@ -568,24 +571,18 @@ typedef struct {
 static void refdb_fs_backend__iterator_free(git_reference_iterator *_iter)
 {
 	refdb_fs_iter *iter = (refdb_fs_iter *) _iter;
-	char *loose_path;
-	size_t i;
-
-	git_vector_foreach(&iter->loose, i, loose_path) {
-		git__free(loose_path);
-	}
 
 	git_vector_free(&iter->loose);
-
-	git__free(iter->glob);
+	git_pool_clear(&iter->pool);
 	git__free(iter);
 }
 
 static int iter_load_loose_paths(refdb_fs_backend *backend, refdb_fs_iter *iter)
 {
+	int error = 0;
 	git_strmap *packfile = backend->refcache.packfile;
 	git_buf path = GIT_BUF_INIT;
-	git_iterator *fsit;
+	git_iterator *fsit = NULL;
 	const git_index_entry *entry = NULL;
 
 	if (!backend->path) /* do nothing if no path for loose refs */
@@ -594,15 +591,16 @@ static int iter_load_loose_paths(refdb_fs_backend *backend, refdb_fs_iter *iter)
 	if (git_buf_printf(&path, "%s/refs", backend->path) < 0)
 		return -1;
 
-	if (git_iterator_for_filesystem(&fsit, git_buf_cstr(&path), 0, NULL, NULL) < 0)
-		return -1;
-
-	git_vector_init(&iter->loose, 8, NULL);
-	git_buf_sets(&path, GIT_REFS_DIR);
+	if ((error = git_iterator_for_filesystem(
+			&fsit, git_buf_cstr(&path), 0, NULL, NULL)) < 0 ||
+		(error = git_vector_init(&iter->loose, 8, NULL)) < 0 ||
+		(error = git_buf_sets(&path, GIT_REFS_DIR)) < 0)
+		goto cleanup;
 
 	while (!git_iterator_advance(&entry, fsit)) {
 		const char *ref_name;
 		khiter_t pos;
+		char *ref_dup;
 
 		git_buf_truncate(&path, strlen(GIT_REFS_DIR));
 		git_buf_puts(&path, entry->path);
@@ -618,9 +616,16 @@ static int iter_load_loose_paths(refdb_fs_backend *backend, refdb_fs_iter *iter)
 			ref->flags |= PACKREF_SHADOWED;
 		}
 
-		git_vector_insert(&iter->loose, git__strdup(ref_name));
+		if (!(ref_dup = git_pool_strdup(&iter->pool, ref_name))) {
+			error = -1;
+			goto cleanup;
 		}
 
+		if ((error = git_vector_insert(&iter->loose, ref_dup)) < 0)
+			goto cleanup;
+	}
+
+cleanup:
 	git_iterator_free(fsit);
 	git_buf_free(&path);
 
@@ -727,20 +732,26 @@ static int refdb_fs_backend__iterator(
 	iter = git__calloc(1, sizeof(refdb_fs_iter));
 	GITERR_CHECK_ALLOC(iter);
 
-	if (glob != NULL)
-		iter->glob = git__strdup(glob);
+	if (git_pool_init(&iter->pool, 1, 0) < 0)
+		goto fail;
+
+	if (glob != NULL &&
+		(iter->glob = git_pool_strdup(&iter->pool, glob)) == NULL)
+		goto fail;
 
 	iter->parent.next = refdb_fs_backend__iterator_next;
 	iter->parent.next_name = refdb_fs_backend__iterator_next_name;
 	iter->parent.free = refdb_fs_backend__iterator_free;
 
-	if (iter_load_loose_paths(backend, iter) < 0) {
-		refdb_fs_backend__iterator_free((git_reference_iterator *)iter);
-		return -1;
-	}
+	if (iter_load_loose_paths(backend, iter) < 0)
+		goto fail;
 
 	*out = (git_reference_iterator *)iter;
 	return 0;
+
+fail:
+	refdb_fs_backend__iterator_free((git_reference_iterator *)iter);
+	return -1;
 }
 
 static bool ref_is_available(