Repository API for creating references

[orirawlings]

A few issues have been reported recently (#481 and #430) that relate to some API confusion around creating new references for a repository.

I'm curious if there is some benefit to providing some high level API in Repository to enable the creation of branches and/or tags.

I threw together this patch to gather some quick feedback on whether this is something beneficial for the project, and to see if the API makes sense. I started with CreateBranch for now. Tests, further documentation, and demo examples are pending.

I figured that sticking to the options pattern for the arguments to CreateBranch would give us the ability to extend this easily in the future to support other features available in the standard git branch tool. Such features include configuring an upstream branches, tracking branches, editing description, etc.

[mcuadros]

This is related to #487

[orirawlings]

Closing for now