On most platforms it's fine to create symlinks to nonexisting files. Not
so on Windows, where the type of a symlink (file or directory) needs to
be set at creation time. So depending on whether the target file exists
or not, we may end up with different symlink types. This creates a
problem when performing checkouts, where we simply iterate over all blobs
that need to be updated without treating symlinks any special. If the
target file of the symlink is going to be checked out after the symlink
itself, then the symlink will be created as directory symlink and not as
file symlink.

Fix the issue by iterating over blobs twice: once to perform postponed
deletions and updates to non-symlink blobs, and once to perform updates
to symlink blobs.