mingw: fix fatal error working on mapped network drives on Windows

In 1e64d18 (mingw: do resolve symlinks in `getcwd()`) a problem was
introduced that causes git for Windows to stop working with certain
mapped network drives (in particular, drives that are mapped to
locations with long path names). Error message was "fatal: Unable to
read current working directory: No such file or directory". Present
change fixes this issue as discussed in
#2480

Signed-off-by: Bjoern Mueller <bjoernm@gmx.de>

compat/mingw.c

@@ -1155,8 +1155,13 @@ char *mingw_getcwd(char *pointer, int len)
 			return NULL;
 		ret = GetFinalPathNameByHandleW(hnd, wpointer, ARRAY_SIZE(wpointer), 0);
 		CloseHandle(hnd);
-		if (!ret || ret >= ARRAY_SIZE(wpointer))
-			return NULL;
+		if (!ret || ret >= ARRAY_SIZE(wpointer)) {
+			ret = GetLongPathNameW(cwd, wpointer, ARRAY_SIZE(wpointer));
+			if (!ret || ret >= ARRAY_SIZE(wpointer)) {
+				errno = ret ? ENAMETOOLONG : err_win_to_posix(GetLastError());
+				return NULL;
+			}
+		}
 		if (xwcstoutf(pointer, normalize_ntpath(wpointer), len) < 0)
 			return NULL;
 		return pointer;