Confusing Cherry Pick Branch Name
### Summary
When I view my list of branches, it is filled with `cherry-pick-XXXXXXX` branches that require a significant amount of investigation to determine if I still need each one.
### Steps to reproduce
1. Merge a merge request
2. Click the "Cherry-pick" button
3. Pick a branch
4. Check "Start a new merge request"
### Example Project
https://gitlab.com/gitlab-org/gitlab-ce/branches?search=cherry-pick
### What is the current *bug* behavior?
The branch is named `cherry-pick-{hash}`. This does not provide any useful information in the branch list.
### What is the expected *correct* behavior?
The branch name should provide context into what branch it came from and what branch it is targeting.
### Relevant logs and/or screenshots

### Output of checks
This bug happens on GitLab.com
### Possible fixes
- Name the branches `{target_branch}-{source_branch}` so that cherry-picking "42-feature-branch" into "stage" becomes "stage-42-feature-branch".
- Allow users to customize the branch name. (See gitlab-ce#29279)
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