Rename the branches directory to stable in accordance with https://sourceforge.net/apps/trac/mingw-w64/wiki/VersionSpec

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/stable/releases/v2_branch@4409 4407c894-4637-0410-b4f5-ada5f102cad1