git merge dev 分支合并到master 如何解决冲突

使用git 分支合并到master主线，但因为同时修改一个文件，导致合并之后文件多出了很多<< 这样的！如何解决冲突？一定要手动编辑冲突文件，去掉那些 << >>>DEV再提交吗？

cd ~
mkdir demo
cd demo
git init
git status // on branch master
echo "first line " >> index.txt 
git add . && git commit -m "first head"
git status // on branch master
git branch // * master
git branch dev
git checkout dev
git branch // * dev ,master 
ls // index.txt
echo "sec line in dev" >> index.txt 
git add . && git commit -m "in branch dev"
git checkout master 
git status // on branch master 
echo "sec line on branch master" >> index.txt
git add . && git commit -m "2"

git merge dev 
Auto-merging index.txt
CONFLICT (content): Merge conflict in index.txt
Automatic merge failed; fix conflicts and then commit the result.

git diff index.txt
diff --cc index.txt
index dda3583,8fa96cd..0000000
--- a/index.txt
+++ b/index.txt
@@@ -1,2 -1,2 +1,6 @@@
  first line 
++<<<<<<< HEAD
 +sec line on branch master
++=======
+ sec line in dev 
++>>>>>>> dev

first line 
<<<<<<< HEAD
sec line on branch master
=======
sec line in dev 
>>>>>>> dev

合并dev 到master，结果这个文件出现了 <<< HEAD >>> dev 这样的，如何将他们合并成功？非得手动去编辑，再提交吗？