Git repository input should fetch the default branch by default

[expipiplus1]

At the moment if no branch is specified it fetches master. https://github.com/NixOS/hydra/blob/master/src/lib/Hydra/Plugin/GitInput.pm#L70

[domenkozar]

@expipiplus1 so if branch is not specified, it should just be omitted?

[expipiplus1]

@domenkozar yes, I think that'll do the right thing.

[ajs124]

Just ran into this. The default is still master (https://github.com/NixOS/hydra/blob/master/src/lib/Hydra/Plugin/GitInput.pm#L31) instead of the actual default branch.

[grahamc]

To fix this we'll need to identify the name of the branch that has been fetched. This is because the name of the branch is an input to the evaluation, and is therefor also an input in to the caching. Ideally the fix for this also comes with a test.