mingw: add fallback for rmdir in case directory is in use

The same logic as for unlink and rename also applies to rmdir. For
example in case you have a shell open in a git controlled folder. This
will easily fail. So lets be nice for such cases as well.

Signed-off-by: Heiko Voigt <heiko.voigt@mahr.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -175,6 +175,9 @@ int link(const char *oldpath, const char *newpath);
 int mingw_unlink(const char *pathname);
 #define unlink mingw_unlink
 
+int mingw_rmdir(const char *path);
+#define rmdir mingw_rmdir
+
 int mingw_open (const char *filename, int oflags, ...);
 #define open mingw_open