cache: Print message when new cache is created

Sometimes, users run restic without retaining the local cache
directories. This was reported several times in the past.

Restic will now print a message whenever a new cache directory is
created from scratch (i.e. it did not exist before), so users have a
chance to recognize when the cache is not kept between different runs of
restic.

cmd/restic/global.go

@@ -376,6 +376,10 @@ func OpenRepository(opts GlobalOptions) (*repository.Repository, error) {
 		return s, nil
 	}
 
+	if c.Created && !opts.JSON {
+		Verbosef("created new cache in %v\n", c.Base)
+	}
+
 	// start using the cache
 	s.UseCache(c)