diffusion.getrecentcommitsbypath fails with 500 error when non existing callsign is passed:
```
>>> UNRECOVERABLE FATAL ERROR <<<
Call to a member function getCommit() on null
```
Expected Behavior:
Return more graceful error notifying caller that such callsign/repository does not exist
Reproduction steps:
Open conduit: https://secure.phabricator.com/conduit/method/diffusion.getrecentcommitsbypath/
Enter:
callsign: "obviouslynotexisting"
path: "/random"
Click call method