Create merge rules to keep old or new whole file

The rules KEEP_OLDER and KEEP_NEWER only keep proper lines on conflict. However, sometimes there could be a line without conflicts and the user prefers to keep the whole file of a specific version rather than try to merge it.

Start by locating the implementation of the KEEP_OLDER and KEEP_NEWER rules and the tests covering conflict resolution. Determine where whole-file selection can be represented for conflict-free lines, then add rules that retain the complete older or newer file and tests showing that the selected version is preserved.

Written by the indexing model from the issue text.