[bug] repect code added when modify on the first or last line

### version
6.5.0

### Environment
windows 7

node v12.16.3
npm v6.14.4
yarn v1.12.1

### Current Behavior
add code on the first or last line, when reinstall, the code is repeated

### Steps to Reproduce

init environment
```
>yarn init
>yarn add -D patch-package postinstall-postinstall
>yarn add axios@0.21.4
```

add `"scripts": { "postinstall": "patch-package" },` to package.json

edit file "node_modules/axios/index.js", add `var aaa = 3;` to the first line
```js
var aaa = 3;
module.exports = require('./lib/axios');
```

save file and run command:
```
>yarn patch-package axios
```

patch-package will then generate the file: axios+0.21.4.patch
```
diff --git a/node_modules/axios/index.js b/node_modules/axios/index.js
index 79dfd09..c995a6f 100644
--- a/node_modules/axios/index.js
+++ b/node_modules/axios/index.js
@@ -1 +1,2 @@
+var aaa = 3;
module.exports = require('./lib/axios');
\ No newline at end of file

```

then remove directory "node_modules", reinstall package
```
>yarn install
```

the new file is (node_modules/axios/index.js)
```js
var aaa = 3;
var aaa = 3;
module.exports = require('./lib/axios');
```

You can see that the modified code appears twice, The same problem occurs when code is added at the end

every time you run `yarn patch-package`

a new code `var aaa = 3;` will be add to the file