PR should mention fixes or closes issue number

When a PR is merged then that PR is closed. However we want the corresponding issue to be closed too at the same time.

In the body of the PR we can mention the issue that this PR closes. This can be done like this. Any of the following two options works.

Fixes #100
Closes #100

If the PR closes multiple issues then all those issues can be mentioned.

Fixes #100, Fixes #103

Github doc has more details on this.