Do not push branch before adding commits

[ddgenome]

Currently when creating commits on a branch, it appears the branch is created and pushed before the commits are added:

https://github.com/atomist/artifact-source/blob/81034cb4c59ff36e816a4a2139bd9e324b75c94c/src/main/scala/com/atomist/source/git/GitHubServices.scala#L84

and then the changes are added to the branch via a commit and then the branch is pushed again.

This causes unnecessary churn in CI and causes issues in downstream handlers for branch builds, e.g., CascadeBuilds in spring-team-handlers.

Can artifact-source be changed to do all the work it needs to do locally and then just make one push at the end?

[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.