Change x265 stable checkout back to mercurial

I discovered that the stable tar.gz for x265 contains a folder with the branch number.
I switched back to mercurial (with an ugly copy-paste, fell free to factorize if possible) but I added the '-r stable' option to get the 'stable' branch.
Merge pull request #46 from gcsx/patch-4

Change x265 stable checkout back to mercurial