Add option to limit snapshots list

[renard]

This patch adds a --limit option to limit snapshots list to the n
last snapshots. It is very similar to the --last one but does not
limit to one entry.

Output example:

$ restic snapshots --limit 2
repository 0d3eb989 opened successfully, password is correct
ID        Time                 Host        Tags        Paths
------------------------------------------------------------
5a33bdcc  2020-12-14 12:30:00  local                   /home
73887d8e  2020-12-15 12:30:00  local                   /home
------------------------------------------------------------
2 snapshots

To keep compatibility with previous versions a new function has been
added: FilterSnapshotsWithLimit. This function that filters a list
of snapshots to only return the N last entries for each hostname and
path. FilterLastSnapshots calls FilterSnapshotsWithLimit(list, 1).

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>

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

--last option limit to the very last snapshot for each hosts. In
some case you want to show the n last snapshots. For example if you
want a weekly report of daily snapshots you cannot have this but by
using shell tools and wont report the correct number of snapshots.

This PR adds a --limit to limit display of the n last snapshots (see
above).

Was the change discussed in an issue or in the forum before?

No.

Checklist

• I have read the Contribution Guidelines
• I have enabled maintainer edits for this PR
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• 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

[renard]

I didn't want to do deep changes with this patch. I am not sure that cobra allows to have flags with optional value.

Ideally it would be better to reuse the --last with following behavior:

• not set: list all snapshosts
• --last: only list last snapshot (current behavor)
• --last 10: only list last 10 snapshots (what `--limit actually does).

Not sure about the test, it seems that snapshots does not have any check but for a regression bug.

[rawtaz]

I agree that reusing the --last option is better than introducing a --limit option.

[renard]

What would be the best option then?

I can convert --last to take an integer argument. I am not sure I can define it to act like I suggested above.
Or I can make it require an argument but this may break existing scripts.

OTOH is it a problem to change FilterLastSnapshots to accept a limit argument? or would it be better to introduce the FilterSnapshotsWithLimit like I did?

Thanks for your feedback.

[MichaelEischer]

It would be possible to get --last=n as parameter type. However, that looks like it could cause a lot of confusion, as we only use that variant for --verbose.

After discussing with @rawtaz we arrived at the following suggestion: Name the parameter --latest n instead of limit and deprecate --last, but make sure that it still works. Cobra supports this via something like f.MarkDeprecated("last", "use --latest 1").

[MichaelEischer]

LGTM. I've removed the man page modification and rebased the commit to the current master branch.