- Update locales

We can look at the PushCommits object to see how many commits were
included in a commit, and add some template logic to only show the
comparison link when there are at least 2 commits in a push. We also
correct the link to display the number of commits.

We have new tab, but we had no entry. That's why it was showing "code"
(lowercase) as this is text id, where we were expecting properly title cased
"Code" to be shown in English version.

Also add Polish translation "code=Kod".

- update locales
- update README

The button appears when a file is viewed in a branch or a tag. It points
to a URL containing the branch's (or tag's) current commit id so that
it'll always point to the same content.