An older merge does not move a source

The forge announces what it finds merged, and an old merge surfacing late moved the recorded head
backwards and rebuilt everything built from that repository, once per old merge. A merge made
before the source was last seen is history; one that says nothing about when is taken as news.
The catalogue now carries when each source was last seen. The bus-records test follows #116:
a module's tools are every one under its own name.

internal/link/events.go

@@ -128,6 +128,8 @@ type SourceMoved struct {
 	Commit   string `json:"merge_commit_sha"`
 	CloneURL string `json:"clone_url"`
 	HTMLURL  string `json:"html_url"`
+	// MergedAt is when the forge merged it, RFC 3339. What decides whether this is news.
+	MergedAt string `json:"merged_at"`
 }
 
 type Upgraded struct {