R1:49570cb1c478

Fix an issue with mention transactions in Calendar

Summary:
Fixes an issue where mentioning an event name (like `E999`) in a revision summary (and probably elsewhere) would produce an error like this:

```
2015/06/08 17:30:22 [error] 27702#0: *307450 FastCGI sent in stderr: "PHP message: [2015-06-08 17:30:22] EXCEPTION: (Exception) Transaction ("PHID-XACT-CEVT-zglgzy36aote5ja", of type "core:edge") requires a handle ("PHID-DREV-4m6vorimvg4bm3ltskca") that it did not load. at…