Implement `hallmark checkout BRANCH` in the `next` branch

[rndsrc]

Currently, hallmark only manipulates the index, but does not actually manage files in Worktree.
Implement hallmark checkout BRANCH so we can download files to Worktree.

[rohinsant]

Hi CK, we were trying to implement the cache folder functionality in the .hm repo. However, we had a couple of questions regarding when the folders inside the cache should be updated. We can think of 2 workflows:

1. The data inside the repository is monitored by a tool (such as Watchdog) that tracks any addition or deletion of files into the repository. In this method, the cache and the data.tsv of the active worktree branch gets updated immediately after a file is changed, added or deleted. The folders in the cache will be created by the information in the data.tsv. In this case, the functionality of the data.tsv file would be modified because it would track the files in the repository independent of the 'hallmark add'.
2. The cache gets updated when a 'hallmark add' function is run. This updates the data.tsv and the folders inside the cache would get created using the information of the SHA1 keys in the data.tsv. If a file was never added, the SHA1 of the file would not show in the cache.
   Which one of these options would be best to implement and is there something else we should consider?
   Thanks!

[rndsrc]

I think method 2 mirror git behavior better. Let's go with 2.

git add actually "add file contents to the index". In this sense, maybe we should call the "cache/" directory "index/" instead?