review: rebuild a file the mesh once wrote whole, keep links, give back a missing line end, and release a hold only after the write (hq issue 128)

internal/declaration/block_test.go

@@ -20,6 +20,7 @@ func TestAFileWrittenIntoABlockIsRefusedUnlessItIsOnlyItsLines(t *testing.T) {
 		"at on a whole file":        {`{"id":"f","type":"file","path":"/etc/hosts","content":"a","at":"start"}`, `at "start"`},
 		"at on a JSON file":         {`{"id":"f","type":"file","path":"/etc/x.json","into":"json","content":"{}","at":"end"}`, `at "end"`},
 		"at somewhere else":         {`{"id":"f","type":"file","path":"/etc/hosts","into":"block","content":"a","at":"middle"}`, `"start" or "end"`},
+		"an id ending in a space":   {`{"id":"f ","type":"file","path":"/etc/hosts","into":"block","content":"a"}`, "whitespace"},
 		"an unknown format":         {`{"id":"f","type":"file","path":"/etc/hosts","into":"lines","content":"a"}`, `"json" or "block"`},
 	} {
 		_, err := Parse([]byte(`{"declaration":1,"resources":[` + c.resource + `]}`))