VALIDATE_ALL_CODEBASE: false fails to detect modified files in GitLab MR pipelines due to bare git diff on detached HEAD

[srikant-sahu]

Describe the bug
When using MegaLinter in a GitLab CI Merge Request pipeline with VALIDATE_ALL_CODEBASE: "false", the engine fails to detect any changed files.
The Updated Sources Reporter outputs No source file has been formatted or fixed and skips all file validations. This happens because MegaLinter's underlying Python process executes a bare workspace diff (git diff --abbrev=40 --full-index -M --raw -z --no-color), which returns zero results in environments using clean detached HEAD checkouts (like GitLab Runner)

To Reproduce
Steps to reproduce the behavior:

1. Create a GitLab CI pipeline targeted at a Merge Request event ($CI_PIPELINE_SOURCE == "merge_request_event").
2. Set VALIDATE_ALL_CODEBASE: "false" and GIT_DEPTH: 0.
3. Push changes to a feature branch and open a Merge Request.
4. Observe the MegaLinter log. It will state that 0 files were updated/found despite active file differences existing between the source and target branch
   Logs from job:
   Start updated Sources Reporter
   sys.platform='linux', git_executable='git'
   Popen(['git', 'diff', '--abbrev=40', '--full-index', '-M', '--raw', '-z', '--no-color'], cwd=/builds/..., stdin=None, shell=False, universal_newlines=False)
   Updated files :
   [Updated Sources Reporter] No source file has been formatted or fixed

Expected behavior
Expect to see the new/modified files in the Updated files output and associated linters to run.

[echoix]

You can’t expect to validate a difference with another branch if you only do a shallow clone of the 0 other previous commits with no other commit information for any other branches either.

Git needs to actually have fetched the commit of the other branch in order to use a diff with that branch. A fixed number of commits instead of 0 will not always work if your branch was based of way back and these number of commits don’t include the branch you’d like to compare to. It is a tradeoff of using shallow clones. https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---depthdepth

There’s no more magic here than any other tools. Super-Linter would either error out or not do it.

[srikant-sahu]

Gitlab MR pipeline does a detached HEAD clone with git depth set to 20 and a git diff shows the changed files. But megalinter doesn't pick up those files. This doesn't work even without GIT_DEPTH var set to 0.