Infrastructure for current directory handling

Infrastructure for current directory handling

Add infrastructure to allow commands to specify if they need a git
repository, if they need to be called from within the working tree,
and if they should cd to the root of the working tree before doing
anything else.

For now, all commands are set to just require a repository (except
"stg clone", which is set to require nothing), which means the only
thing that's added is some very light error checking. The idea is to
tighten this for commands that turn out to need it.

Signed-off-by: Karl Hasselström <kha@treskal.com>