Allow commit messages to close several issues at once

This PR improves how commit message can close issues.

Until now, we were able to close single issues in commit message using some keywords. But closing several issues at once seemed impossible.

With this patch, we can mix keywords and issues # in commit message to close issues. For example:

```
git commit -m"Awesome commit message (Fix #20, Fixes #21 and Closes #22) This commit is also related to #17 and fixes #18"
```

Using such commit message will close `#20`, `#21`, `#22` and `#18`. `#17` won't be closed as it does not match the pattern. It also works with multiline commit messages.

http://feedback.gitlab.com/forums/176466-general/suggestions/5673472-allow-multiple-issues-to-be-closed-from-a-commit-m
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