backup: print full snapshot id in JSON summary

[MichaelEischer]

What does this PR change? What problem does it solve?

The JSON output only included the shortened ID of the snapshot and not the full ID. The shortened ID could be ambiguous in rare cases and always has to be expanded first before restic can use it. As the JSON output is intended to be machine readable, there is no use in shortening the ID before printing it.

Strictly speaking, this change is not backwards compatible, but I don't expect much fall-out. In the worst case users have to explicitly truncate the ID down to 8 characters.

Was the change previously discussed in an issue or on the forum?

Fixes #2724

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[rawtaz]

Makes sense, no need to shorten it in programmatic/structured output.

[MichaelEischer]

Hmm, the forget and snapshots command print both the full ID and a ShortID. But introducing a LongID here is just horribly inconsistent and not really useful.

[rawtaz]

The forget command prints snapshot IDs like this:

"id": "89cefc5d61f95b66dc36e25029d54582fdb5d92d159af95dbb89cd4458f3c4a5",
"short_id": "89cefc5d"

The snapshots command prints snapshot IDs like this:

"id": "ea55667d08c38695cc6d2367ac50f9bc66b7b88a946427dcadde76b806820b07",
"short_id": "ea55667d"

The backup command currently prints snapshot IDs like this:

"snapshot_id": "ea55667d"

We can simply change the backup command to output the snapshot ID as id (long format) and short_id (short format) to make it consistent with the others, while for now keeping snapshot_id as-is (but making a note that it is deprecated in the changelog and the code) and remove that in the future. This should make it all consistent and fully backwards compatible.

[rawtaz]

@fd0 Did you read my comment above?

[MichaelEischer]

I still prefer the current change of just outputting the full snapshot id. In the context of the backup command, it's probably not really obvious, that id would be the snapshot id (it can't really be something else, but still). And replacing the existing field complicates things a lot.

[fd0]

I'm with @MichaelEischer, just print the full ID in the JSON field.