Convert usernames in commit comments

Sometimes the mediawiki comments have usernames, which we can map to @ style GitHub usernames. e.g.

```
Reverted edits by [[Special:Contributions/xxx|xxx]] ([[User talk:xxx|talk]]) to last revision by [[User:Peter|Peter]]
```

-->

```
Reverted edits by @xxx to last revision by @peterjc
```

As a bonus this is much shorter so the logs look nicer.
