Windows: correct detection of EISDIR in mingw_open()

According to the Linux open(2) man page, open() must return EISDIR
if a directory was attempted to be opened for writing. Our emulation
in mingw_open() does not get this right: it checks only for O_CREAT.

Fix it to check for a write request.

This fixes a failure in reflog handling, which opens files with
O_APPEND|O_WRONLY, but without O_CREAT, and expects EISDIR when the
named file happens to be a directory.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -312,7 +312,7 @@ int mingw_open (const char *filename, int oflags, ...)
 		return -1;
 	fd = _wopen(wfilename, oflags, mode);
 
-	if (fd < 0 && (oflags & O_CREAT) && errno == EACCES) {
+	if (fd < 0 && (oflags & O_ACCMODE) != O_RDONLY && errno == EACCES) {
 		DWORD attrs = GetFileAttributesW(wfilename);
 		if (attrs != INVALID_FILE_ATTRIBUTES && (attrs & FILE_ATTRIBUTE_DIRECTORY))
 			errno = EISDIR;