[git] How can I copy the content of a branch to a new local branch?

I have worked on a local branch and also pushed the changes to remote. I want to revert the changes on that branch and do something else on it, but I don't want to lose the work completely. I was thinking of something like create a new branch locally and copy the old branch there, then I can revert the changes and continue working on the old branch. Is there a better way maybe? Or how do I do this?

With Git 2.15 (Q4 2017), "git branch" learned "-c/-C" to create a new branch by copying an existing one.

See commit c8b2cec (18 Jun 2017) by Ævar Arnfjörð Bjarmason (avar).
See commit 52d59cc, commit 5463caa (18 Jun 2017) by Sahil Dua (sahildua2305).
^{(Merged by Junio C Hamano -- gitster -- in commit 3b48045, 03 Oct 2017)}

branch: add a --copy (-c) option to go with --move (-m)

Add the ability to --copy a branch and its reflog and configuration, this uses the same underlying machinery as the --move (-m) option except the reflog and configuration is copied instead of being moved.

This is useful for e.g. copying a topic branch to a new version, e.g. work to work-2 after submitting the work topic to the list, while preserving all the tracking info and other configuration that goes with the branch, and unlike --move keeping the other already-submitted branch around for reference.

Note: when copying a branch, you remain on your current branch.
As Junio C Hamano explains, the initial implementation of this new feature was modifying HEAD, which was not good:

When creating a new branch B by copying the branch A that happens to be the current branch, it also updates HEAD to point at the new branch.
It probably was made this way because "git branch -c A B" piggybacked its implementation on "git branch -m A B",

This does not match the usual expectation.
If I were sitting on a blue chair, and somebody comes and repaints it to red, I would accept ending up sitting on a chair that is now red (I am also OK to stand, instead, as there no longer is my favourite blue chair).

But if somebody creates a new red chair, modelling it after the blue chair I am sitting on, I do not expect to be booted off of the blue chair and ending up on sitting on the new red one.

git branch copyOfMyBranch MyBranch

This avoids the potentially time-consuming and unnecessary act of checking out a branch. Recall that a checkout modifies the "working tree", which could take a long time if it is large or contains large files (images or videos, for example).