We are going to use git2go (see next patch) for which canonical import
path is git (import "github.com/libgit2/git2go" results in package name
being autotruncated to just "git") so free up the "git" name for that
package.

Reason is: git() - as function - is used not often, while the package
will be used often.

Regarding naming: not sure it is good choice but ggit() is something
like xgit(), only g is for "GitError".