fix async commit, add/commit use file.base path

As a result of this patch, it's possible to `add()` and `commit()` in repositories other than that of the current working directory.  I'd like to see this behavior everywhere, TBH, but the other calls accept a `{cwd: "..."}` option, so a change is not strictly necessary.

This also allows one to chain `commit()` using the `gulp.task` callback.  It's a hack, but it seemed to be the simplest possible fix and therefore, the easiest with which to explain the problem.

Thanks for gulp-git!