mingw: move Git for Windows' system config where users expect it

Git for Windows' prefix is `/mingw64/` (or `/mingw32/` for 32-bit versions), therefore the system config is located at the clunky location `C:\Program Files\Git\mingw64\etc\gitconfig`. This moves the system config into a more logical location: the `mingw64` part of `C:\Program Files\Git\mingw64\etc\gitconfig` never made sense, as it is a mere implementation detail. Let's skip the `mingw64` part and move this to `C:\Program Files\Git\etc\gitconfig`. Side note: in the rare (and not recommended) case a user chooses to install 32-bit Git for Windows on a 64-bit system, the path will of course be `C:\Program Files (x86)\Git\etc\gitconfig`. Background: During the Git for Windows v1.x days, the system config was located at `C:\Program Files (x86)\Git\etc\gitconfig`. With Git for Windows v2.x, it moved to `C:\Program Files\Git\mingw64\gitconfig` (or `C:\Program Files (x86)\Git\mingw32\gitconfig`). Rather than fixing it back then, we tried to introduce a "Windows-wide" config, but that never caught on. Likewise, we move the system `gitattributes` into the same directory. Obviously, we are cautious to do this only for the known install locations `/mingw64` and `/mingw32`; If anybody wants to override that while building their version of Git (e.g. via `make prefix=$HOME`), we leave the default location of the system config and gitattributes alone. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>