In short, files that end with "adfc_diagram" file type are metadata files to keep track of element position in an ADF task flow. ADF task flow offers a process-flow like visual panel for developers to re-position the elements (page/fragment, router, Java bean methods, etc). The file ending with "adfc_diagram" is the one tracking the positions of all elements so that next time the task flow is open the saved visual positions are presented. Another blog spot on the same matter: http://javalopez.blogspot.com/2015/03/should-i-commit-adfcdiagram-files.html
To attain knowledge, add things everyday. To attain wisdom, remove things every day.