Allow repo:branch placeholder syntax to omit repo

[stuarteberg]

Right now one can use special syntax to avoid hard-coding a UUID in a url, such as:

http://my-dvid-server.janelia.org:8000/api/node/abc123:master/segmentation/info

...where abc123 is ANY uuid in the repo I'm interested in. Internally, DVID will replace abc123:master with the UUID corresponding to the tip of the master branch.

If there's only a single repo on the server, then it's a little inconvenient to supply the abc123 part. For single-repo servers, can the following format be supported instead?

http://my-dvid-server.janelia.org:8000/api/node/:master/segmentation/info