Merge branch 'graphs-branch-switching' into 'master'

Add ref switching support for graphs

It is now possible to view contributor and commit statistics for
refs beyond the default branch. While the Git ref already was part of
the path it has not been used before.

Additionally, the code in RefsController#switch has been slighliy
refactored since it would have become confusing with additional if-else
branches added.

See merge request !932