Fix listing of modified files

[vkucera]

Consider the merge-base commit as the starting point of diff instead of listing all differences between branches.

Fixes #2125

Proposed Changes

MegaLinter currently gets the list of files to check by comparing HEAD and the base branch.

• That is fine for pull_request events where HEAD is the merge commit and git diff only lists files in the PR.
• It is not fine for push events where the base branch can contain commits not present in the head branch and git diff also lists files from those commits.

To list only files from commits in the head branch which are not present in the base branch, one should run git diff against the common ancestor (aka merge-base) of the two branches.
This can be done with git diff <base-branch>... which is equivalent to:

• git diff <base-branch>...HEAD
• git diff --merge-base <base-branch> HEAD
• git diff $(git merge-base <base-branch> HEAD) HEAD

Readiness Checklist

Author/Contributor

• Add entry to the CHANGELOG listing the change and linking to the corresponding issue (if appropriate)
• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance

[vkucera]

The test failed after 5e1adac because of a missing merge base which I fixed by adding fetch-depth: 0 in the checkout step of that action.
Now the test is failing at test_get_linter_help but I don't understand the issue.

[nvuillam]

it seems everything is working :)

We'll merge this one once the current CI issue with eslint-plugin-json is fixed :)

[vkucera]

it seems everything is working :)

We'll merge this one once the current CI issue with eslint-plugin-json is fixed :)

Thanks @nvuillam , which issue are you referring to?

[nvuillam]

@vkucera it's fixed, i merge once CI is ok (and it should be ^^ )