t: make alternates test work on Windows
t: make alternates test work on Windows

This test works fine on Unix systems, but fails on Windows. This occurs
because the MSYS environment uses Unix-style paths, but Git wants
Windows-style paths in its environment and alternates files. Convert the
paths to an appropriate format to ensure this test passes on Windows as
well.

github/git-lfs,github/git-lfs,github/git-lfs