It is not exactly a new feature, but I find myself using the --sort option of git branch a lot more often than before these days.
The way I work is that I review patches that were mailed-in during previous night (my time) in the morning. For each promising new topic, I decide where the topic should eventually be merged (some are fixes that should go to older maintenance tracks, some are new features that we will not want to merge to the maintenance tracks), create a dedicated topic branch for it, apply these patches, re-review them once more and then test the changes in isolation. Each existing topics that is redone in response to previous reviews is handled the same way. Its branch is rewound and the new round of patches are applied instead.
After accumulating the new and updated topics that way without integrating with anything else, I'd often forget how many topics need to be integrated into the test branches (i.e. jch and pu), and I can do this:
$ git branch --no-merged pu --sort=-committerdate
This lists the topic branches that are not part of pu, which is the branch that is supposed to contain all the testable things, and sort them according to the commit date (i.e. the time I last touched it) of the tip of the topic branch. There often are topics that were once picked up, but turned out to be not ready even for the pu branch, and left around without getting merged to anywhere as a reminder for myself (otherwise, I'll forget pinging their authors about them), and they will sink in the older part of the output, while the freshly created and updated ones will float to the top of the output. This reminds me of the topics from the day that I need to reintegrate before starting the integration testing.
The --sort option appeared first in Git 2.7.0.
Another command that I use often these days is Michael Haggerty's when-merged script, available in his repository at GitHub. After finding a problematic line in the source and identifying the exact commit that introduced the line by using git blame, I can see when it landed in the mainline by doing this:
$ git when-merged $that_problematic_commit master | git name-rev --stdin
This gives the merge commit that brought in the commit as part of a topic to the mainline, and after that, it is just the matter of turning it into a revision name to find the oldest maintenance track that needs to be fixed, which is partially done by passing its output through the name-rev filter.