Added co2.sh

Script does git checkout of a certain file of a certain branch or tag into
another directory.

It does what git checkout-index should had done, if that command would take
a branch as an argument. If such command exists, this might be obsoleted.

Category: development
Fixing ugly output & non-working -d

Checking out from local directory looks ugly if "./" is
appended. Only difference is what is printed out however, functionality is the
same.

Issue with -d was an actual bug. Trivial so included it in this patch.

Category: bugfix
Added warning for duplicate checkout

Checking out what you already have out gives option to reject or continue.

Category: development