Make `GIT_CHECKOUT_SAFE` the default.  `NONE` is never what the userwants _by default_; people expect checkout to, well, check things out.Instead, it should be an opt-in "dry run" mode.This removes some odd code in internal callers of `checkout` that takesa `git_checkout_options` and updates the mode to `SAFE`.  This is nowunnecessary since everything has better defaults.

Instead of dealing with the clone options sub-options (fetch, checkout,etc) individually, treat them as a cohesive whole when passing themthroughout the system.Additionally, move some functions around within the file to avoidunnecessary decls at the top of the file.  And change a functionsignature to avoid conflating truth with error.