Tagging is probably something you want to do, but if you want something in your commit log, the following may work:
You could use the no fast-forward tag when merging your branch. This adds a specific commit for the merging, and you could add 'finish task 1' to the commit message. So for instance you would do:
git merge task-1 --no-ff
And then you have a clear indication of where this branch was merged in.
I hope I understood your question properly, and this helps you out.