git: centralize git cli operations

Move all of the git command line logic into a single object, inspired by
the object already in buildx.

The basic implemenation allows for configuring a git cli for a specific
repository, along with various authorization settings and custom
binaries. Commands can be run for that repository, and a few helpers are
provided for accessing data on it - more to come in the future
hopefully.

Signed-off-by: Justin Chadwell <me@jedevc.com>