Support fetching changelog from bitbucket-server

[zeldigas]

What would you like Renovate to be able to do?

Right now repositories stored in BitbucketServer are not processed during release-notes fetching, I got the following debug message in logs:

DEBUG: Fetching changelog: https://mybb.acme.com/projects/PRJ/repos/internal-repo/browse (3.2.0 -> 3.3.0) (repository=~jsmith/changelog-test)
DEBUG: Repository URL does not match any known github hosts - skipping changelog retrieval (repository=~jsmith/changelog-test)
       "manager": "maven",
       "depName": "com.acme.libs:my-lib",
       "sourceUrl": "https://mybb.acme.com/projects/PRJ/repos/internal-repo/browse"

In case of configured platform to bitbucket-server I'd like renovate to support going to repository by resolved url and fetch release notes from CHANGELOG.md if it's located in repo.

Did you already have any implementation ideas?

Looks like current code responsible for fetching changelog is full of if checks for platform - for github and gitlab. I think that adding one more if branch in all the places would increase entropy significantly. I think that better option would be to introduce some interface that can be implemented by various platforms.

I can try to work on this, but might need some help from renovate dev team in case of questions as code base for PR resolution is not that small + relies on concept of releases that some platforms do not have (bitbucket-server specifically) that would affect the way to identify the range of looked up items in changelog.md file.

[viceice]

I think we have an open issue for supporting other platform changelog fetching. That code needs a major refactoring before we can add new platforms for sure.

[HonkingGoose]

I think I found the issue that @viceice is talking about, and I think the current issue is a duplicate of the older issue.

Duplicate of #3903

[viceice]

Duplicate of #3903