cmd/coordinator: put the sub-repo name and revision in the log heading

Change-Id: I6b6ac6edbdfa46e91e507b2225e7aa33e97c0ffe
Reviewed-on: https://go-review.googlesource.com/10961
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/coordinator/coordinator.go b/cmd/coordinator/coordinator.go
index 7c8cc68..4809a35 100644
--- a/cmd/coordinator/coordinator.go
+++ b/cmd/coordinator/coordinator.go
@@ -1253,7 +1253,11 @@
 
 	execStartTime := time.Now()
 	st.logEventTime("pre_exec")
-	fmt.Fprintf(st, "%s at %v\n\n", st.name, st.rev)
+	fmt.Fprintf(st, "%s at %v", st.name, st.rev)
+	if st.isSubrepo() {
+		fmt.Fprintf(st, " building %v at %v", st.subName, st.subRev)
+	}
+	fmt.Fprint(st, "\n\n")
 
 	var remoteErr error
 	if st.conf.SplitMakeRun() {