gitattributes: Implement the use of --source
Since Git 2.40 git-check-attr(1) learned to accept a --source argument to specify the tree-ish to read the file from. This makes it unnecessary to copy the contents of that file to info/attributes.
This change makes CheckAttr() accept a git revision to read the .gitattributes straight from the given revision.
Label: maintenance::refactor
Edited by Toon Claes