Fix getting prompted to enter a user name and email when rebasing even though valid user name and email exists.

The problem is that gitdll.c::get_windows_home_directory() would hold a pointer to the home environment variable which would later get blown away causing the current static pointer to be pointing at garbage memory.

This problem manifested itself in the rebase dialog. When the user chose to rebase, the user would get prompted with "User name and email must be set before commit. Do you want to set these now?" even though the user had the user name and email set properly.

Run the following on a repository that could be rebased:

/command:rebase /path:C:\Path\To\Repo\That\Could\Get\Rebased

Hit "Start Rebase" or "Fast Forward" and you will be prompted with a prompt letting you know that you need to enter user name and email even though you don't need to.

Note: I am not 100% sure this is the correct fix to this issue but this is stab #1 (closed) at coming up with a working resolution.

Also... this appears to be caused by a fairly recent modification to the TGit repository: 6abcae1e