Try to fix snapshot publishing

[lazar-mitrovic]

This is a draft PR that tries to tackle fixing of snapshot publishing. I replaced jgit dependency with shell invocation of git executable itself in order to workaround issues with public keys on existing back-ends. We can do this since it is reasonable to assume that every machine that runs this code has git properly setup.

There seem to be several task scheduling issues that prevent this PR from "just working". They exhibit as deleteRecursively command failing several times in a row, and other tasks continuing to run when cloneSnapshot task fails even when --no-parallel option is set. My guess is that some task dependencies and inputs/outputs are incorrectly setup but ATM I don't have enough time to debug this. If anyone has some ideas, please yell.