How to find the branch from commit id
On GitHub specifically, you now can see the branch a given commit is part of.
The blog post "Branch and Tag Labels For Commit Pages" details:
If the commit is not on the default branch, the indicator will show the branches which contain the commit.
If the commit is part of an unmerged pull request, a link will be shown.
Once the commit makes it to the default branch, any tags that contain the commit will be shown, and the default branch will be the only branch listed.
Original answer
You can list those branches:
git branch --contains <commit>
# in your case
git branch --contains <commit> | grep x
More details in "Git: Finding what branch a commit came from".
Don't forget that a commit can be part of several branches.
Just a cheat.
You can see the branch on the commit detail page of Github like the following:
https://github.com/{github_username}/{repo}/commit/{commit_id}
Ex: https://github.com/chungth/Laravel-5.2-Bootstrap3-starter-site/commit/f81c13234a9da988613e14e52f752abec5f51997
You can see the current branch name on that page.