osmo-depcheck: don't use /tmp, better git code

* replace --gitdir with --workdir and give it a new folder structure:
  * git/$repo: downloaded source code
  * build/$repo: files created during the build process
  * install/: installation prefix
* adjust the jenkins job to use --workdir
* fetch --tags when source exists already
* readable error message for failed git checkout

Change-Id: I06589277b9d54a2af177451cfab2ca1a658b4058
Relates: OS#2642
5 files changed