Merge branch 'jk/no-diff-emit-common'

"git merge-tree" used to mishandle "both sides added" conflict with
its own "create a fake ancestor file that has the common parts of
what both sides have added and do a 3-way merge" logic; this has
been updated to use the usual "3-way merge with an empty blob as
the fake common ancestor file" approach used in the rest of the
system.

* jk/no-diff-emit-common:
  xdiff: drop XDL_EMIT_COMMON
  merge-tree: drop generate_common strategy
  merge-one-file: use empty blob for add/add base

git-merge-one-file.sh

@@ -120,8 +120,7 @@ case "${1:-.}${2:-.}${3:-.}" in
 	case "$1" in
 	'')
 		echo "Added $4 in both, but differently."
-		orig=$(git-unpack-file $2)
-		create_virtual_base "$orig" "$src2"
+		orig=$(git-unpack-file e69de29bb2d1d6434b8b29ae775ad8c2e48c5391)
 		;;
 	*)
 		echo "Auto-merging $4"