Enable having a footer like the KeepAChangelog one

[MarcoIeni]

Is your feature request related to a problem? Please describe.
In my changelogs, I would like to have a footer like the one of KeepAChangelog:

[Unreleased]: https://github.com/olivierlacan/keep-a-changelog/compare/v1.0.0...HEAD
[1.0.0]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.3.0...v1.0.0
[0.3.0]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.2.0...v0.3.0

This enables you to click on the links of each entry of the changelog to see the changes.

I think this isn't possible to achieve at the moment with git-cliff.

Have you ever thought about supporting such a use case?

Describe the solution you'd like

There should be a way for git-cliff to update the footer of the changelog together with the body.

Describe alternatives you've considered
git-cliff could have a new entry in the configuration: version_link, in which the user can specify a template to add after the [<version>]: row.

For example, in the configuration you can have something like:

version_link = """
{% if previous %}\
{% if previous.version %}\
    https://github.com/olivierlacan/keep-a-changelog/compare/{{ previous.version }}...{{ version }}
{% else %}\
    https://github.com/olivierlacan/keep-a-changelog/releases/tag/{{ version }}
{% endif %}\
{% endif %}\
"""

Then, git-cliff takes care of updating the versions at the end of the changelog.

Maybe you could also have a unreleased_link, because the template in this case is different:

unreleased_link = """
    https://github.com/olivierlacan/keep-a-changelog/compare/{{ version }}...HEAD
"""

Additional context
Enabling "templated footers" isn't enough because every time you add a new entry to the changelog, you need to edit the [Unreleased] row with the latest version.

For example, the footer for the 0.3.0 version looks like this:

[Unreleased]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.3.0...HEAD
[0.3.0]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.2.0...v0.3.0

Instead the footer of the 1.0.0 version looks like this:

[Unreleased]: https://github.com/olivierlacan/keep-a-changelog/compare/v1.0.0...HEAD
[1.0.0]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.3.0...v1.0.0
[0.3.0]: https://github.com/olivierlacan/keep-a-changelog/compare/v0.2.0...v0.3.0

As you can see we have both added the [1.0.0] row, and edited the [Unreleased] one.

[link2xt]

I have tried adding this in changelog.body:

{% if version %}\
{% if previous.version %}\
[{{ version | trim_start_matches(pat="v") }}]: https://github.com/deltachat/deltachat-core-rust/compare/{{ previous.version }}..{{ version }}\n
{% endif %}\
{% endif %}

I do not really care about all the links being at the end of file, if they get into the end of section it is fine too.
Then running git cliff -l works as expected.

But unfortunately it does not work when I want to generate a changelog for a version not released yet. If I run git cliff -u -t v1.131.5, I get this:

[1.131.5]: https://github.com/deltachat/deltachat-core-rust/compare/v1.131.5..v1.131.5

I expected previous.version to be set to the latest existing tag.

[orhun]

I expected previous.version to be set to the latest existing tag.

Fixed this bug in 6203f77

[link2xt]

I expected previous.version to be set to the latest existing tag.

Fixed this bug in 6203f77

Great, thanks!

[orhun]

This will be supported when #369 is merged.