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

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>