Git Bash: Use the link file name to add "MINGW32" / "MSYS" to the window title

"MINGW32" / "MSYS" was missing form the title since ec4de09, and it never
played nicely with the "Git Bash" part of the title, but now it does.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>