Hi ,
Thanks to community feedback, we have updated GitHub to include tags for Preview releases which match to changelist of binary Preview releases from Launcher.
For example: https://github/EpicGames/UnrealEngine/tree/4.7.0-preview-6 (you must be signed in to GitHub to view)
Details here: https://.unrealengine/showthread.php?58706-Preview-release-tags-available-on-GitHub
Cheers!