git-p4: Fix p4 user cache population on Windows.

Fall back to USERPROFILE if HOME isn't set.

Signed-off-by: Simon Hausmann <shausman@trolltech.com>
Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/contrib/fast-import/git-p4 b/contrib/fast-import/git-p4
index e3404ca..1f5a56e 100755
--- a/contrib/fast-import/git-p4
+++ b/contrib/fast-import/git-p4
@@ -901,7 +901,8 @@
                            % (labelDetails["label"], change))
 
     def getUserCacheFilename(self):
-        return os.environ["HOME"] + "/.gitp4-usercache.txt"
+        home = os.environ.get("HOME", os.environ.get("USERPROFILE"))
+        return home + "/.gitp4-usercache.txt"
 
     def getUserMapFromPerforceServer(self):
         if self.userMapFromPerforceServer: