FR: Show summary of what changed (files added, modified...) when the working copy is snapshotted by a jujutsu command

[AngelEzquerra]

Is your feature request related to a problem? Please describe.
Jujutsu automatically snapshots (i.e. updates) the working copy commit as you use it. It does that even when you use commands that on traditional VCSs would not update any commit (e.g. jj status or jj log). While this is great and a big part of what makes jujutsu appealing, users coming from other VCSs might not realize that this is happening.

Describe the solution you'd like
It'd be great if whenever a jujutsu operation performs a working copy snapshot, it showed a brief summary of what was updated, specially the addition of new files to the repo. For example, if running jj log updated 2 files and added 3 new files into the new working copy snapshot, jujutsu could print the following information on the console: "Working copy snapshot made: 3 files added, 2 files updated". Note that this message would indicate what changed in the snapshot, not the actual snapshot contents (e.g. in the example, jj status might show 10 files modified and 7 added).
Ideally it should be possible to disable this message through a config flag, but I think it would be better to enable it by default.
The benefit of this change is that it would make it easier for new users to understand how jujutsu works. In addition to that it would make it less likely that a user would accidentally add a (potentially secret) file to the repo, which could later be shared with others when pushing that changeset.

Describe alternatives you've considered
A (similar) alternative would be to only indicate when new files are tracked. This would be less "noisy" and cover the main reason why I feel that this new feature would be useful.

[PhilipMetzger]

This will be extremely noisy as snapshots are extremely frequent and if you add watchman with the core.watchman.register_snapshot_trigger = true option not even user-controlled.

[AngelEzquerra]

@PhilipMetzger, do you think it would be very noisy if it was limited (by default) to file additions? That is my biggest concern.

[PhilipMetzger]

This will just make it dependent on repo-size and their respective additions, which makes the tool not consistent in my opinion.

[arxanas]

This will be extremely noisy as snapshots are extremely frequent and if you add watchman with the core.watchman.register_snapshot_trigger = true option not even user-controlled.

How frequently will it be? Isn't it basically once per command (excluding the Watchman situation)? Are you saying that's too noisy?

This will just make it dependent on repo-size and their respective additions, which makes the tool not consistent in my opinion.

How would it be dependent on the repo size if it's reporting changes since the last working copy snapshot?

[yuja]

I don't know if that would be noisy, but it's a valid point that if user had auto-snapshotting enabled or had background jj process, the output wouldn't be predictable.

I personally don't want to see snapshot stats because I use the @ commit as a working-copy in traditional VCSs. jj st == hg st, and I'm not interested in whether the status is updated by the last jj invocation.

[AngelEzquerra]

@yuja, my main concern is with added files. That is the main difference in behavior (and thus the must surprising thing) compared to mercurial.
I think many users coming from other VCSs will not realize (at first!) that doing jj st will add new files to the working copy commit. This proposal is intended to educate those users and make it less likely that files are added to the repo (and later shared) accidentally due to the auto-tracking.
I think the creation of new files should be rare enough that if this message were limited to additions (at least by default) it would not be noisy in most cases.

[AngelEzquerra]

Thinking a bit more about this, maybe when using jj st, which already shows the list of added files, this would not be needed. I am more concerned about other commands which are entirely "passive", in other VCSs but which do not show the status, such as jj log, jj config, etc. When using those commands which implicitly track new files, giving some sort of feedback that that happened to the user would be very useful to avoid mistakes, IMHO.
If this info were not shown on jj status, this would be much less noisy, I think.