git_operations.py should be as machete-agnostic as possible

[PawelLipski]

In particular, I'm not happy about rebase calling machete-pre-rebase hook, which is a git-machete-specific thing. TBD what else should me moved away from git_operations.py (Ctrl+F for machete).

[AABur]

@PawelLipski needs clarification.

machete is found:

• GIT_MACHETE_EDITOR
• GIT_MACHETE_REBASE_OPTS
• machete-pre-rebase

What's wrong with them?

[PawelLipski]

Huh nothing wrong specifically with the above... it's a matter of general design soundness: having well-separated layers so that the lower layers (git_operations) don't know anything about the higher layers (client, which has Machete-specific stuff). I'd suggest moving methods up to MacheteClient and/or passing stuff (GIT_MACHETE_REBASE_OPTS?) as parameters to git_operations methods