GitGraph: commit options for merge commits

[WeeBull]

This is really a reopening of #2934 as the original request was for merge to support all fields that commit does, but the change only added support for tags.

Is your feature request related to a problem? Please describe.
Merge commits should be treated like ordinary commits, in that they should be customize-able with the same syntax. e.g.

commit tag: "vx.x.1" type: HIGHLIGHT id: "1"
merge develop tag: "vx.x.1" type: HIGHLIGHT id: "2"

Describe the solution you'd like
The syntax tree for merge should probably just be exactly the same as the commit tree after the branch name.

Additional context
Right now it is impossible to refer to a merge commit. The case I'm struggling with is cherry-picking a merge. Adding an extra commit afterwards isn't the same thing.

[marijadjukic]

I was just writting a documentation with Git diagram and naturally added id option to a merge commit to be able to cherry-pick it and was disappointed when I realized it is not supported. Cherry-picking of merged requests is verry common in development so not being able to visualize it is a big gap. It would be really nice if this can be supported.

[amyq]

I can confirm the lack of the id: parameter on merge commits is a problem. The only option I've found so far is a hacky misuse of the tag: parameter to add words to a merge commit.

Similarly, some merge commits are squash merges, and there's no easy way to show the difference. That's why I'm (for now) abusing the tag: parameter to note that the merge commit is a squash merge.