Fails to switch branches properly

[jhgoodwin]

Bug description

In our org, we're using nbgitpuller using the url links to pull code for users to use. One helpful facility is the branching option to pull a specific branch.

The problem is that the way nbgitpuller uses git log to find the delta between your current branch and origin fails to work if you SWITCH branches.

My proposal is to ensure the git log command compares the CURRENT branch to origin, instead of right now, it compares the TARGET branch to origin.

Expected behaviour

• Use link with branch option to pull a feature branch
• Observe it works
• Use same link without branch option (defaults to master)
• Observe it works

Actual behaviour

• Use link with branch option to pull a feature branch
• Observe it works (current behavior)
• Use same link without branch option (defaults to master)
• Errors similar to screenshot above

How to reproduce

See expected/actual above.

In addition, I read the code in master for pull.py. The git log command at the command line will fails, even outside nbgitpuller and for the same reason.

Your personal set up

• OS: Linux
• Version: jupyterhub 0.9.0
• Configuration: Openshift 3.11, nbgitpuller 0.8.0