packaging: Modified set_version.sh script:

- Do no longer modify the version string of a checked out tag (e.g. XtreemFS-unstable).
- Fixed setting the version string for branches checkouts.
- Fixed detection of the release name in -i mode.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2915 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534