Git: How to find out on which branch a tag is?
Even shorter:
git branch --contains tags/<tag>
(it works for any tree-ish reference)
If you can find which commit a tag refers to:
git rev-parse --verify tags/<tag>^{commit} # or, shorter: git rev-parse tags/<tag>~0
Then you can find which branch contain that commit.
git branch --contains <commit>
As commented below by user3356885, for the fetched branches (branches in remotes namespace)
git branch -a --contains tags/<tag>git branch -a --contains <commit>