Fix testMerge in test_git

This fixes two small issues:

1. Creating a new branch using the branch method of GitPlugin creates a new branch but does not switch to that branch. However, the test assumes that the new branch is activated. There an according call to switchBranch is made.
2. Some checks in the merge tests were useless because a local variable "files" hides an outer variable of the same name. This change renames the inner variables to "lsfiles".