Added support for non-symlink checkouts on Windows when elevated rights are missing

[fkollmann]

On Windows, elevated rights are required for symlinks.

If they are missing, which is true for almost all users, the library will fail with "A required privilege is not held by the client." (ERROR_PRIVILEGE_NOT_HELD).

This change will handle the error and write the file, instead of trying to create a symlink.

[mcuadros]

This is how behaves git on windows?

[fkollmann]

Got on Windows behaves in two different ways:

a) Developer Mode: Git will create Symlinks
b) Normal Mode: Git will simply create a file containing the file path (basically creating useless garbage)

For case (a) there is an ongoing fix at Go: golang/go#22874
For case (b) this PR will produce a more reasonable behavior, as the file will get the correct content (on clean checkout, only)

See https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/ for technical details

[fkollmann]

I will close this PR in favor of an implementation that mimics the behavior of Go on Windows: #776