Assert the exact amount of links we expect. While there, check that a
plain git_clone() automatically chooses to link.

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).

When passed the LINK_FILES flag, the recursive copy will hardlink files
instead of copying them.

A call like git_clone("./foo", "./foo1") writes origin's url as './foo',
which makes it unusable, as they're relative to different things.

Go with git's behaviour and store the realpath as the url.

When git is given such a path, it will perform a "local clone",
bypassing the git-aware protocol and simply copying over all objects
that exist in the source.

Copy this behaviour when given a local path.

Fixed miscellaneous documentation errors.

We go through the different backends in order, so it's not an error if
at least one of the backends has the data we want.