Fix GetAffectedFiles failed when encountering a null old commit

[lng2020]

When using protected/unprotected file patterns, git push --set-upstream xxx xxx will cause an internal server error.

This bug is because when git push --set-upstream, a null commit is passed to function GetAffectedFiles as oldCommitID, but git diff cannot compare null commits. So when oldCommit is null, we should modify the oldCommitID to empty tree commit("4b825dc642cb6eb9a060e54bf8d69288fbee4904") to use git diff.

[wxiaoguang]

Wait, is it right to get all changed file between current commit and the empty tree commit?

[KN4CK3R]

I think the solution is ok but maybe the check should not be in that method but some frames up the callstack.

Edit: The solution could be a problem because this will now scan the whole repo for (dis)allowed files and that may conflict with already existing files which are not modified in the new branch. So it may be better to use the diverging commit id with the default branch and only fallback to the empty tree commit if there is none.

[wxiaoguang]

The solution could be a problem because this will now scan the whole repo for (dis)allowed files and that may conflict with already existing files which are not modified in the new branch.

Yup, I think so too.

And I recalled this PR (not fully related but the "empty sha" behavior should be explicitly clarified)

• Support getting changed files when commit ID is EmptySHA #26290

[lunny]

The solution could be a problem because this will now scan the whole repo for (dis)allowed files and that may conflict with already existing files which are not modified in the new branch.

Yup, I think so too.

And I recalled this PR (not fully related but the "empty sha" behavior should be explicitly clarified)

* [Support getting changed files when commit ID is `EmptySHA` #26290](https://github.com/go-gitea/gitea/pull/26290)

Maybe we need to check all possbile places which contains oldcommit and newcommit and handle them like #26290 did.

[silverwind]

Merge this? Edit: Nevermind, i see the blocked.

[lunny]

This should have been replaced by #26290 . @Zettat123 @lng2020

[lunny]

Close as outdated.