Git merge conflict to always take the newest file

I think you will have to write your own merge-driver for this. See "git help gitattributes", the "Defining a custom merge driver" sections for details on how to do just that.

I came up this little merge driver that does what I want. For my purpose it is hard coded to the "master" branch and to the "origin" remote. I do not know how to make these parts dynamic.

#!/usr/bin/env sh
if git merge-file -p -q "$2" "$1" "$3" > /dev/null;
        then git merge-file "$2" "$1" "$3";
        else
                MINE=$(git log --format="%ct" --no-merges master -1);
                THEIRS=$(git log --format="%ct" --no-merges origin/master -1);
                if [ $MINE -gt $THEIRS ];
                        then git merge-file -q --ours "$2" "$1" "$3";
                        else git merge-file -q --theirs "$2" "$1" "$3";
                fi
fi

In short I look for the last commit with git-log that was not a merge, formatted as UNIX timestamp, then I compare them and run a custom git-merge with eiter ours or their version.

As a little bonus, it first makes a check to see if it is possible to merge the file without conflict. If that is possible, it merges both files.