Fix drive mounted directory on Windows

On my Windows machine, I mount drive D: to the directory C:\src.

The old implementation returns the incorrect 'C:\\??\\Volume{ad2eb15e-f293-4d48-a448-54757d95a97c}' result, which breaks the repo init command.

With the use of os.path.realpath, it can return 'D:\\' correctly.

Change-Id: Ia5f53989055125cb282d4123cf55d060718aa1ff
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/416580


Reviewed-by: Greg Edelston <gredelston@google.com>
Tested-by: Kaiyi Li <kaiyili@google.com>
Commit-Queue: Kaiyi Li <kaiyili@google.com>