Replacing checks for '/' in branch names by checks for "origin/"

[johanneslerch]

The problem described in #101 seems to be caused by several locations checking if a branch name contains a forward slash '/' to identify remote branches. However, this over-approximating check makes it impossible to illustrate, e.g., a git flow using branch names like "feature/foobar".
In my changes I replaced the check for '/' by a check for "origin/", which seems to solve the problem. However, this solution will again fail as soon as a remote is not called origin, so this is also not the best solution.
It can be tested already on the fork: https://johanneslerch.github.io/visualizing-git/

[johanneslerch]

Re-created this pull request using a separate branch to avoid mixing several features into a single pull request. Please, consider #106