Changelog fixes

[knapsu]

Third pack of fixes and improvements from Housemarque.

• Fix changelog calculations. Branch is now taken into account when showint incremental changes.
• Fix changelog calculations when Plastic SCM server is in different timezone than Jenkins server.
• Fix changelog calculation when building on Jenkins Agents.
• Improvement Show last changeset info instead of nothing when rerunning old build.
• Fix Avoid workspace name conflicts. Use paths for workspace identification instead of names.
• Improvement Show Plastic SCM changeset information in Job status view.
• Improvement Plastic SCM build data view.