feat: Allow checking out scratch area on target revisions

[DiamondJoseph]

Fixes #813 (@noemifrisina for comment on current implementation while I work on tests).

Allows configuring a target revision for a repository checked out in the scratch area.

I think I have overcomplicated the actual use case, but hopefully this diagram helps explain: checks out an existing branch of the desired name (if possible), else checks out the existing tag (if possible) and creates a branch of the name to track changes.

flowchart TD
    Start{Repo already exists in scratch?} --> |No| Clone[Clone default branch of repo from remote]
    Start --> |Yes| Connect[Use existing repo on current branch]
    Clone --> Configured{Target revision configured?}
    Connect --> Configured
    Configured --> |No| End
    Configured --> |Yes| Fetch[Fetch configured remote]
    Fetch --> Branch{Branch of name exists?}
    Branch --> |Yes| Checkout[Check out branch]
    Branch --> |No| Exists{Tag configured?}
    Exists --> |Yes| Tag{Tag of name exists?}
    Exists --> |No| Except
    Tag --> |Yes| Create[Check out tag]
    Create --> CB[Create branch of name]
    Tag --> |No| Except
    Checkout --> ShouldPull{.pull?}
    CB --> Checkout
    ShouldPull --> |No| End
    ShouldPull --> |Yes| pull
    pull --> End

Loading

example:

      - name: "dodal"
        remote_url: https://github.com/DiamondLightSource/dodal.git
        targetRevision:
          tag: 1.65.0
          branch: dev

Checks out dodal on 1.65 and creates a dev branch to track it, if a branch called dev does not exist locally