[Bugfix] Fixed "file=null"-bug when exporting new translations

Awesome bundle, really. I found a bug though... 

### To reproduce error
My default language is "sv" and i support translations on "sv" and "en". 

```` yaml
//messages.sv.yml
a: "foo_sv"
b: "bar_sv"
c: "baz_sv"
````

```` yaml
//messages.en.yml
a: "foo"
b: "bar"
````

Run the input command. Add the missing translation with key "c" (Do not use the "new translation" link). Run the export command. 

**Result** 
```` yaml
//messages.en.yml
a: "foo"
b: "bar"
````
**Expected result**
```` yaml
//messages.en.yml
a: "foo"
b: "bar"
c: "baz"
````

### The problem
The translation row in the database do not get a file property. (file_id = null). 

This PR solves that issue by looking for an other Translation object by the same TransUnit and get the file from there.