Handle scenario when a local branch's tracking branch is deleted

Technically, this bug is with GitPython. How to reproduce:

1. create a local branch that tracks a remote branch
2. delete the remote branch now
3. do the following:

import git
repo = git.repo('/path/to/repo')
branch = repo.branches[0] 
# assuming that the local branch is the first one
print (branch.tracking_branch().object.hexsha)

This fails. *We should probably check if the tracking branch really exists? *