Windows sessions ignore drive letters

One follow up observation and an additional problem based on the following correspondence table:
| Windows Dir | ignore_string |
|---------|---------------|
| c:\TEMP | "c:/TEMP"     |
| d:\TEMP | "d:/TEMP"     |

On Windows machines, it looks like `vim-prosession` does not care about the drive-letter, and saves the session into `%TEMP.vim` regardless of where GVIM is launched.