Rename VcsCommand to VcsCommandTag, Command into VcsCommand
(Too generic to not mistake for Core::Command IMNSHO),
remove the now unneeded namespace qualification when not needed,
adjust surrounding code.

Change-Id: Iceb18a21e5e6dffa1a622241286f766985bb8d22
Reviewed-by: Christian Stenger <christian.stenger@digia.com>