exp apply: safeguard against unclean workspaces

dvc exp apply will clear the workspace of untracked files and unstaged modifications. To prevent users losing work, it would be good to follow git’s convention of preventing overwriting of untracked/modified files unless the user gives explicit permission.

• If dvc exp apply is executed on an unclean workspace, fail with a suggestion to stash work or specify a --force flag
• dvc exp apply --force acts like the current command does.

[dberenbaum]

@pmrowla Forcibly overwriting conflicts is intended as explained here and in https://github.com/iterative/dvc.org/issues/2397, but it seems like experiments are overwriting everything in the workspace, even untracked files. This seems to conflict with https://github.com/iterative/dvc.org/issues/2397#issuecomment-822887190.

Example:

$ dvc stage add -n mystage echo foo
Creating 'dvc.yaml'
Adding stage 'mystage' in 'dvc.yaml'

To track the changes with git, run:

    git add dvc.yaml

To enable auto staging, run:

        dvc config core.autostage true
$ dvc exp run
Running stage 'mystage':
> echo foo
foo
Generating lock file 'dvc.lock'
Updating lock file 'dvc.lock'

To track the changes with git, run:

    git add dvc.lock dvc.yaml

To enable auto staging, run:

        dvc config core.autostage true

Reproduced experiment(s): exp-396c5
Experiment results have been applied to your workspace.

To promote an experiment to a Git branch run:

        dvc exp branch <exp> <branch>

$ touch newfile
$ dvc exp apply exp-396c5
Changes for experiment 'exp-396c5' have been applied to your current workspace.
$ git status
On branch main
Untracked files:
  (use "git add <file>..." to include in what will be committed)
        dvc.lock
        dvc.yaml

nothing added to commit but untracked files present (use "git add" to track)
$ ls
dvc.lock dvc.yaml

[mattlbeck]

@pmrowla I hadn’t noticed the --no-force argument. On reflection, switching between experiments would necessitate use of a --force flag if it existed, and I guess that use-case is considered the most common.

I will monitor how I use exp apply for now, and whether I can easily encorporate --no-force into my workflow.