mingw: ensure `getcwd()` reports the correct case

mingw: ensure `getcwd()` reports the correct case

When switching the current working directory, say, in PowerShell, it is
quite possible to use a different capitalization than the one that is
recorded on disk. While doing the same in `cmd.exe` adjusts the
capitalization magically, that does not happen in PowerShell so that
`getcwd()` returns the current directory in a different way than is
recorded on disk.

Typically this creates no problems except when you call

	git log .

in a subdirectory called, say, "GIT/" but you switched to "Git/" and
your `getcwd()` reports the latter, then Git won't understand that you
wanted to see the history as per the `GIT/` subdirectory but it thinks you
wanted to see the history of some directory that may have existed in the
past (but actually never did).

So let's be extra careful to adjust the capitalization of the current
directory before working with it.

Reported by a few PowerShell power users ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>