changed: Link to the PR that introduced a new feature

[dlang-bugzilla-migration]

Transferred from: https://issues.dlang.org/show_bug.cgi?id=18122

User @wilzbach reported (2017-Dec-24 22:22:50):

Would be very interesting for people who want to know the details. In most cases the changelog every is added as part of the PR, so it should be possible to query this directly from the git log in the tools/changed.d script.