Add ability to quickly copy commit message from existing commit
### Problem to solve
I often need to copy the commit message of an existing commit. Currently, this isn't a super smooth process - I have to open up the commit details, copy the commit summary ("Add reusable project_selector component" in the screenshot below), and then separately copy the commit body ("This commit adds a reusable UI component..."). The commit summary is also hard to select without also selecting my avatar picture.

It would be great to be able to copy this message with a single click, similar to how one can copy a commit SHA:

### Target audience
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
### Further details
I would use this feature as part of my workflow when force pushing to a branch:
- I create an MR and ask for feedback.
- Someone takes a look and leaves some suggestions.
- I update my branch with the suggestions, but I want to roll my updates into my previous commit. So, I do the following:
- Make my changes on my branch
- "Undo" the last commit: `git reset --soft HEAD~1`
- Copy the commit message from GitLab.com using the somewhat painful process described in the **Problem to solve** section
- Re-commit my changes with the same message as before: `git add --all`, `git commit -m "<previous commit message here>"`
- Rebase my branch to incorporate any new changes: `git fetch`, `git rebase origin/master`
- Update the remote branch: `git push --force`
Whether or not this is a best practice is debatable :smile:, but there may also be other use cases for copying a commit's message.
### Proposal
Add a button that allows the entire commit message to be copied with a single click.
issue