`TreeDiffEntry` template method `summary()`

[Jollywatt]

It is currently possible to obtain a diff summary like this, using the summary() template method of TreeDiff:

❯ jj log -r @ --no-graph --template "diff.summary()"
M docs/conflicts.md
R docs/design/{run.md => walk.md}
D docs/filesets.md

This PR adds a summary() method to TreeDiffEntry, to allow things like:

❯ target/debug/jj log -r @ --no-graph --template 'diff.files().map(|f| f.summary()).join("\n")'
docs/conflicts.md
docs/design/{run.md => walk.md}
docs/filesets.md

This template method is useful for tools like https://github.com/idursun/jjui which rely on templates to produce output. There, a template like

❯ jj log -r @ --no-graph --template 'diff.files().map(|file| separate(":",
	file.status(),
	file.path(),
	file.summary(),
)).join("\n")'
modified:docs/conflicts.md:docs/conflicts.md
renamed:docs/design/walk.md:docs/design/{run.md => walk.md}
removed:docs/filesets.md:docs/filesets.md

could be used to obtain both the machine readable path and the { => }-style summary together.

Checklist

If applicable:

• [x] I have updated CHANGELOG.md
• [x] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added/updated tests to cover my changes

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[martinvonz]

It looks like you haven't signed the Google CLA yet. We cannot accept he PR without it.

[yuja]

If the goal is to get formatted (or combined) source/target path instead of source().path()/target().path() pair, it might be better to add .display_copied_path() -> String or something.

[martinvonz]

FYI, there may be some changes in this area for the new copy tracking design because the data model supports multiple files being merged into one.

[Jollywatt]

Good points. I will change the name to .copied_path_summary() or similar if you want.