config_file: duplicate accessors for readonly backend

While most functions of the readonly configuration backend are
implemented separately from the writeable configuration backend, the two
functions `config_iterator_new` and `config_get` are shared between
both. This sharing makes it necessary to have some shared data
structures, which is the `diskfile_header` structure. Unfortunately, this
makes the backends harder to grasp than necessary due to all the casting
between structs and also quite error prone.

Reimplement those functions for the readonly backends. As readonly
backends cannot be refreshed anyway, we can remove the calls to
`config_refresh` in there.

src/config_file.c

@@ -284,6 +284,25 @@ out:
 	return error;
 }
 
+static int config_iterator_new_readonly(
+	git_config_iterator **iter,
+	struct git_config_backend *backend)
+{
+	diskfile_header *bh = GIT_CONTAINER_OF(backend, diskfile_header, parent);
+	git_config_entries *entries = NULL;
+	int error;
+
+	if ((error = git_config_entries_dup(&entries, bh->entries)) < 0 ||
+	    (error = git_config_entries_iterator_new(iter, entries)) < 0)
+		goto out;
+
+out:
+	/* Let iterator delete duplicated entries when it's done */
+	git_config_entries_free(entries);
+	return error;
+}
+
+
 static int config_set(git_config_backend *cfg, const char *name, const char *value)
 {
 	diskfile_backend *b = GIT_CONTAINER_OF(cfg, diskfile_backend, header.parent);
@@ -361,6 +380,28 @@ static int config_get(git_config_backend *cfg, const char *key, git_config_entry
 	return 0;
 }
 
+static int config_get_readonly(git_config_backend *cfg, const char *key, git_config_entry **out)
+{
+	diskfile_header *h = GIT_CONTAINER_OF(cfg, diskfile_header, parent);
+	git_config_entries *entries = NULL;
+	git_config_entry *entry;
+	int error = 0;
+
+	if ((entries = diskfile_entries_take(h)) == NULL)
+		return -1;
+
+	if ((error = (git_config_entries_get(&entry, entries, key))) < 0) {
+		git_config_entries_free(entries);
+		return error;
+	}
+
+	entry->free = free_diskfile_entry;
+	entry->payload = entries;
+	*out = entry;
+
+	return 0;
+}
+
 static int config_set_multivar(
 	git_config_backend *cfg, const char *name, const char *regexp, const char *value)
 {
@@ -642,12 +683,12 @@ static int config_snapshot(git_config_backend **out, git_config_backend *source)
 	backend->header.parent.readonly = 1;
 	backend->header.parent.version = GIT_CONFIG_BACKEND_VERSION;
 	backend->header.parent.open = config_readonly_open;
-	backend->header.parent.get = config_get;
+	backend->header.parent.get = config_get_readonly;
 	backend->header.parent.set = config_set_readonly;
 	backend->header.parent.set_multivar = config_set_multivar_readonly;
 	backend->header.parent.del = config_delete_readonly;
 	backend->header.parent.del_multivar = config_delete_multivar_readonly;
-	backend->header.parent.iterator = config_iterator_new;
+	backend->header.parent.iterator = config_iterator_new_readonly;
 	backend->header.parent.lock = config_lock_readonly;
 	backend->header.parent.unlock = config_unlock_readonly;
 	backend->header.parent.free = backend_readonly_free;