lintian-brush: lib.rs: Use new Fixes-lintian-tag commit tag

When people want to manually fix issues reported by lintian, it would be a great help if they could (easily) search for commits by others which already fixed that lintian issue.

Therefor introduce the Fixes-lintian-tag tag, so that the Fixes tag as used by several upstream projects like Linux:

Fixes: <commit ID> ("<commit summary>")

remains 'reserved' for that particular purpose.

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Discussed earlier today and yesterday in #debian-qa on how to make it easier for newcomers to find out how to fix a particular issue that's reported by Debian, I brought up that it would be great if you could see an (list of) examples/commits which show how a particular tag was fixed (instead of the long list of other packages which also do it 'wrong').
When I suggested to use Fixes-lintian-tag: <lintian-tag> it was brought to my attention that lintian-brush does something similar. For the reason stated above, I prefer to keep the 'normal' Fixes-tag for its original (?) purpose and thus introduce a new tag.

I've already applied that to https://salsa.debian.org/diederik/mumble/-/commits/update-to-1.5.634/ (but that branch may disappear when merged).

But having more examples of fixes which can easily be found by searching for it, would be most welcome.