Do not push branch before adding commits

[atomist-attic]

Do not push branch before adding commits

[alankstewart]

@ddgenome Yes, artifact-source can be changed to do all work locally and make one push at the end. However, we will need to switch to JGit as the GitHub V3 api requires the sha of the branch to commit files to it, and if the branch doesn't exist, it needs to be created first.

Using JGit means we no longer have to use the github-java library as well as the extra Rest api calls currently we need to make.

@cdupuis After confirming this with you, I will make a start on converting to JGit.

[alankstewart]

A new JGitServices provides a method, createBranchFromChanges to create a branch and push changes in one commit

[alankstewart]

Callers can use JGitServices#createBranchFromChanges if required.