If requested, git_clone_local_into() will try to link the object files
instead of copying them.
This only works on non-Windows (since it doesn't have this) when both
are on the same filesystem (which are unix semantics).
Cloning from an empty repo must set master's upstream to origin's
master, even if neither of them exist.
Fetching from a non-empty origin must then mark the master branch
for-merge. This currently fails.
Clone callbacks can return non-zero values to cancel the clone.
This adds some tests to verify that this actually works and updates
the documentation to be clearer that this can happen and that the
return value will be propagated back by the clone function.
