Remote paths: canonicalize UNC paths on Win32

Git for Windows will handle UNC paths only when in forward-slash
format, eg "//server/path".  When given a UNC path as a remote,
rewrite standard format ("\\server\path") into this ridiculous
format.
Merge pull request #2619 from ethomson/remotes_with_unc

Remote paths: canonicalize UNC paths on Win32