Don't remove GIT_TRACE environment variable if it's set to a file path

[wilbaker]

Currently VFSForGit removes all GIT_TRACE environment variables when calling git.exe (to avoid breaking the parsing of git output).

https://github.com/Microsoft/VFSForGit/blob/06793f215847ad3023977380d0623cbf59b6cde1/GVFS/GVFS.Common/Git/GitProcess.cs#L409

It would be helpful for diagnosing issues if GIT_TRACE was not removed if it's set to a file path.