How does the "Reset" option in Code Repositories work?
The "Reset" option is equivalent to a git reset where all uncommitted changes of that branch is discarded and the branch is reset to the latest commit.
Is there a way to preview or build a transform based on a historical version of the input dataset?
Specifically in Code Repositories, there is no way to select a specific input transaction (see the existing Input class parameters). However, you can call the createBranch endpoint which takes a transactionRid parameter that can be set to a previous transaction. Then you would be able to use this branch in your transform (branch being a parameter of the Input class).
Additionally, the simplest way to work with historical data is to use Contour: You can very easily change the input dataset version to a previous one, see the docs.