added method to checkout a commit (without options)
checkout the index and discard working dir changes
comments
minor correction
added reference checkout
move detached head to really checkout a commit
move head to to checkout either branch or a detached head
implemented resetting single files to a specific revision and in turn the force checkout
removed GITWRAP_DEFAULT_TLSRESULT according to refactoring
checkout with decision flag to update head or not
check on valid object in updateHEAD as well
Merge pull request #17 from macgitver/feature/checkout

feature: checkout commands