Commit limits

Prevent timeouts when creating/viewing Merge Requests with many
commits. Also, reduce the number of commits shown in the UI from 500
to 100.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14031

This MR avoids Unicorn timeouts in some places and takes 4.5 seconds
off load times in others, when manually testing with a 4500-commit
compare/MR in the linux repo. It does not limit the number of Commit
objects instantiated in memory, just the amount of HTML generated. It
seems that having 4500 commits objects is not a problem in itself. If
it ever becomes one we could do something for Commit objects like we
did for Diff (introduce a CommitCollection or something).

See merge request !3095