If shards.yml was updated with not changes to the dependency resolution, the modification time of shards.yml is newer than that of shards.lock and/or the install dir (.crystal/shards) even after running shards update.
This breaks mtime-based freshness detection (for example used by make). See #380 for details.
shards update should update the modification time of both shard.lock and .crystal/shards even if it didn't change anything. Ditto for shards install.

/cf #380 (comment) f.