Closes https://github.com/gitlabhq/gitlabhq/issues/9265

All inputs of type `search` will have the gray background
and search icon centered.
Because the search magnifier is a bg image,
I had to hide it with `[value=""]`.
I added a little javascript to make sure each input always has it's
own value.

Closes #12677

* Use commit objects instead of IDs when generating diffs
* Use proper references when generating MR's source and target
* Update broken specs

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Now it returns the fully-formatted message so we can be consistent about
how it's shown.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Somehow this test existed on EE but not in CE, so it started failing
after a bad CE-to-EE merge.

Now we apply syntax highlighting to the whole old and new files.
This basically help us to highlight adequately multiline content.

Closes #5908