* fix git switch branches in combination with depth
* the old implementation is correct, but relies on git working correctly
(as only newer versions do)
* with some older git version (e.g. 1.8.3), git fetch remote branch does
not work, if the branch does not yet exist locally
* this patch works around that without explicitly checking the git version
* future refactoring is needed to reduce the number of fetches to a
minimum; but this patch makes the code "correct"
* add git version requirements to doc
* replace set-branches with git 1.7.1 compatible version
281398fdc2