Incorrect branch name displayed in notifications

Currently testion extracts the branch name from `head` proeprty of the pygit2's Repository object after checking out [(link to code)](https://github.com/lablup/testion/blob/78220a8/testion/reporter/base.py#L226-L234), but sometimes those branch names are **wrong**.

Example:

Let's check out why this happens and fix up.
Note that if the last commit is a merge commit, then both 'master' and other branch may point the same commit --- we should check if this applies in our cases as well.