Implement jj next/prev

[dbarnett]

There should be a pair of commands jj next and jj prev for quickly checking out the next/previous commit from @, similar to the next/prev commands from git-branchless.

[dbarnett]

For now, you can add these aliases to your jj config to get something close:

[alias]
prev = ["co", "@--"]
next = ["co", "@-+ ~ @"]

but with a pretty big misfeature that in branch/merge cases instead of getting a picker to say which prev/next rev you want, it just blows up with an error about "more than one revision".

[martinvonz]

I agree. FYI, @rslabbert talked (on Discord) about possibly implementing an interactive command for selecting a commit to check out or edit. Having prev and next that are non-interactive in the linear case still seems useful, though.

[dbarnett]

Oh, another small usability improvement for "more than one revision": It wouldn't hurt anything to have the error list those actual multiple revisions it found, would it? Like:

$ jj next
Error: Revset "@-+ ~ @" resolved to more than one revision:
67318df2df48 tutorial: mention `jj edit` and when it's useful
a83a0415aed8 Do not use "..." between diff chunks when it only replaces 1 line of the diff

[martinvonz]

Oh, another small usability improvement for "more than one revision": It wouldn't hurt anything to have the error list those actual multiple revisions it found, would it? Like:

$ jj next
Error: Revset "@-+ ~ @" resolved to more than one revision:
67318df2df48 tutorial: mention `jj edit` and when it's useful
a83a0415aed8 Do not use "..." between diff chunks when it only replaces 1 line of the diff

It would probably not hurt anything to include up to, say, 5 commits there.

If we wanted to format and color those commits as we usually, I'm not sure how to do it cleanly. So far, the errors we return have a single, plain-text error message, and an optional plain-text hint. We render the text in red and the hint in blue (by default). That happens here:

jj/src/cli_util.rs

Lines 1481 to 1487 in 4a889b9

	Err(CommandError::UserError { message, hint }) => {
	ui.write_error(&format!("Error: {}\n", message)).unwrap();
	if let Some(hint) = hint {
	ui.write_hint(&format!("Hint: {}\n", hint)).unwrap();
	}
	1
	}

. We could of course print out the unwanted revisions before returning the error, which would end up looking more like this:

$ jj next
Hint: Revset "@-+ ~ @" resolved to more than one revision: 
67318df2df48 tutorial: mention `jj edit` and when it's useful
a83a0415aed8 Do not use "..." between diff chunks when it only replaces 1 line of the diff
Error: Revset "@-+ ~ @" resolved to more than one revision.

It might be better to just render it as plain-text error plus hint then, pretty much as you suggested above:

$ jj next
Error: Revset "@-+ ~ @" resolved to more than one revision.
Hint: The revset resolved to these revisions:
67318df2df48 tutorial: mention `jj edit` and when it's useful
a83a0415aed8 Do not use "..." between diff chunks when it only replaces 1 line of the diff

[ilyagr]

Following the idea of #878 (comment), I like

[revset-aliases]
next = "@-+ ~ @" 
prev = "@--"  # Though I usually just type @--

Then, you can do jj up next, jj log -r next (especially if it resolves to multiple revisions), jj rebase -d next or even jj rebase -d next+, etc. You can do this instead of or in addition to the [alias]-es.