Add git commit id to full CEYLON_VERSION

[CeylonMigrationBot]

[@quintesse] It would be very nice that once we have the OBR we include the HEAD commit id of the code that was used to build the distribution. That way we can be sure exactly which compiler somebody uses (especially useful for dealing with non-released versions).

Maybe there's some ideas we could use here: http://stackoverflow.com/questions/3442874/in-git-how-can-i-write-the-current-commit-hash-to-a-file-in-the-same-commit

[Migrated from ceylon/ceylon-common#58]