Use the diff3 merge function in the merge command

Having introduced the `Merge::Diff3` module in the previous commit, we
now use it in `Merge::Resolve` to merge the blobs of conflicting edits
to the same file, replacing the simplistic method of concatenating the
two versions and letting the user figure it out.

lib/merge/resolve.rb

@@ -1,3 +1,5 @@
+require_relative "./diff3"
+
 module Merge
   class Resolve
 
@@ -69,22 +71,15 @@ def merge_blobs(base_oid, left_oid, right_oid)
       result = merge3(base_oid, left_oid, right_oid)
       return result if result
 
-      blob = Database::Blob.new(merged_data(left_oid, right_oid))
-      @repo.database.store(blob)
-      [false, blob.oid]
-    end
+      oids  = [base_oid, left_oid, right_oid]
+      blobs = oids.map { |oid| oid ? @repo.database.load(oid).data : "" }
+      merge = Diff3.merge(*blobs)
 
-    def merged_data(left_oid, right_oid)
-      left_blob  = @repo.database.load(left_oid)
-      right_blob = @repo.database.load(right_oid)
+      data = merge.to_s(@inputs.left_name, @inputs.right_name)
+      blob = Database::Blob.new(data)
+      @repo.database.store(blob)
 
-      [
-        "<<<<<<< #{ @inputs.left_name }\n",
-        left_blob.data,
-        "=======\n",
-        right_blob.data,
-        ">>>>>>> #{ @inputs.right_name }\n"
-      ].join("")
+      [merge.clean?, blob.oid]
     end
 
     def merge_modes(base_mode, left_mode, right_mode)

test/command/merge_test.rb

@@ -187,6 +187,23 @@ def assert_index(*entries)
     end
   end
 
+  describe "unconflicted merge: in-file merge possible" do
+    before do
+      merge3(
+        { "f.txt" => "1\n2\n3\n" },
+        { "f.txt" => "4\n2\n3\n" },
+        { "f.txt" => "1\n2\n5\n" })
+    end
+
+    it "puts the combined changes in the workspace" do
+      assert_workspace "f.txt" => "4\n2\n5\n"
+    end
+
+    it "creates a clean merge" do
+      assert_clean_merge
+    end
+  end
+
   describe "unconflicted merge: edit and mode-change" do
     before do
       merge3(