Eager-load image blob data in diffs

Since gitlab_git 8.0, blob data are lazy-loaded so we have to call
`blob.load_all_data!(repo)` to eager-load them.

Fixes #13458.

/cc @jacobvosmaer @dblessing @rspeicher 

### New image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.18](/uploads/01c2afa2d25f3249b27744154ab2069b/Screen_Shot_2016-02-16_at_11.28.18.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.56](/uploads/5ef72f717f22dfd0c1efa9e6d8ed926f/Screen_Shot_2016-02-16_at_11.30.56.png)

### Replaced image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.20](/uploads/39bb61851608643a86d9d7616139fc6c/Screen_Shot_2016-02-16_at_11.28.20.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.59](/uploads/27e754fe8b57a838ac695bace27618c3/Screen_Shot_2016-02-16_at_11.30.59.png)

### Deleted image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.22](/uploads/3648dcc2c36bc644556080e203a29fca/Screen_Shot_2016-02-16_at_11.28.22.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.01](/uploads/2585915d994bb6e245dd8c8553730bbf/Screen_Shot_2016-02-16_at_11.31.01.png)

### Image diff in merge request diff view

#### Before
![Screen_Shot_2016-02-16_at_11.28.37](/uploads/a7963f7139fc91975ca40568570ce4f0/Screen_Shot_2016-02-16_at_11.28.37.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.04](/uploads/6773dbf3b870f082f0262febc9722d20/Screen_Shot_2016-02-16_at_11.31.04.png)

See merge request !2830