fixup! Arcyd: sanitize git abstraction
Tidy, split abdt_git branch class to new component

To make dependencies more explicit and the source a bit smaller, move
the _ManagedBranch implementation out into it's own component.

Test Plan: $ ./precommit.sh
Tidy, re-organise abdt_branch

Make abdt_branch easier to work with by re-ordering the methods to
put the accessors first and group things a little more logically.

Test Plan: $ ./precommit.sh