prevent deadlock in List() for B2 when b2.connections=1

This is a fix for the following situation (gh-1188):

List() grabs a semaphore token upon entry, starts a goroutine, and
does not release the token until the routine exits (via a defer).

The goroutine iterates over the results from ListCurrentObjects(),
sending them one at a time to a channel, where they are ultimately
processed by be.Load().

Since be.Load() also needs a token, this will result in deadlock if
b2.connections=1.

This fix changes List() so that the token is only held during the call
to ListCurrentObjects().

internal/backend/b2/b2.go

@@ -307,18 +307,17 @@ func (be *b2Backend) List(ctx context.Context, t restic.FileType) <-chan string
 
 	ctx, cancel := context.WithCancel(ctx)
 
-	be.sem.GetToken()
-
 	go func() {
 		defer close(ch)
 		defer cancel()
-		defer be.sem.ReleaseToken()
 
 		prefix := be.Dirname(restic.Handle{Type: t})
 		cur := &b2.Cursor{Prefix: prefix}
 
 		for {
+			be.sem.GetToken()
 			objs, c, err := be.bucket.ListCurrentObjects(ctx, be.listMaxItems, cur)
+			be.sem.ReleaseToken()
 			if err != nil && err != io.EOF {
 				return
 			}