* remove unused variables
* fetch branch name instead of HEAD
fix#3782, which was introduced by f1bacc1d3f9578f26d4ae2f66112cbb2509a7fe8
* disable git depth option for old git versions
fixes#3782
git support for `--depth` did not fully work in old git versions (before 1.8.2)
fall back to full clones/fetches on those versions
* raise required git version to 1.9.1 for depth option
* use correct depth argument in switch_version