This commit uses Git plumbing to reconstruct a merged conflict file
that includes diff3 conflict markers, and then extracts those markers
to replace the traditional two- or three-way conflict with the BASE
revision in the middle pane.

The addition of the '==== BASE ====' string is, while ugly, necessary
in order to actually guarantee some kind of conflict. Without a unique
string there, it's entirely possible that the differences between the
panes will show up as, for example, an insertion from LOCAL to BASE and
a modification between BASE and REMOTE.