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

- Removes looking up authors/committers by name
- Renames `User.find_for_commit` to `User.find_by_any_email`

As of Ruby 1.9, this is the correct way to handle `respond_to?` for
methods implemented by `method_missing`.

See https://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding

Except for Note, which still overrides it.

Now there is a single source of information for which attribute a model
uses to be referenced, and its special character.

Prior, comparison would use the Ruby object's ID, which got out of sync
after a Spring fork and would result in erroneous test failures.

Now we just check that the compared object is a Commit and then compare
their underlying raw commit objects.

and more readable.

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

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

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

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

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

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

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

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

Previously, only number of changed files mattered. Now, number of lines to render in the diff are also taken into account.

A hard limit is set, above which diffs are not rendered and users are not allowed to override that. This prevents high server
resource usage with huge commits.

Related to #1745, #2259

In addition, handle large commits for MergeRequests and Compare controllers.

Also fixes a bug where diffs are loaded twice, if user goes directly to merge_requests/:id/diffs URL.

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