getcwd(mingw): handle the case when there is no cwd

A recent upstream topic introduced checks for certain Git commands that
prevent them from deleting the current working directory, introducing
also a regression test that ensures that commands such as `git version`
_can_ run without a current working directory.

While technically not possible on Windows via the regular Win32 API, we
do run the regression tests in an MSYS2 Bash which uses a POSIX
emulation layer (the MSYS2/Cygwin runtime) where a really evil hack
_does_ allow to delete a directory even if it is the current working
directory.

Therefore, Git needs to be prepared for a missing working directory,
even on Windows.

This issue was not noticed in upstream Git because there was no caller
that tried to discover a Git directory with a deleted current working
directory in the test suite. But in the microsoft/git fork, we do want
to run `pre-command`/`post-command` hooks for every command, even for
`git version`, which means that we make precisely such a call. The bug
is not in that `pre-command`/`post-command` feature, though, but in
`mingw_getcwd()` and needs to be addressed there.

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

compat/mingw.c

@@ -1127,6 +1127,10 @@ char *mingw_getcwd(char *pointer, int len)
 	}
 	if (!ret || ret >= ARRAY_SIZE(wpointer))
 		return NULL;
+	if (GetFileAttributesW(wpointer) == INVALID_FILE_ATTRIBUTES) {
+		errno = ENOENT;
+		return NULL;
+	}
 	if (xwcstoutf(pointer, wpointer, len) < 0)
 		return NULL;
 	convert_slashes(pointer);