add merge request command

To implement

Bigger implementations

• create (from issue??) closes issue?
• checkout|co -- checks out the PR locally (does it git fetch first)? takes PR ID, or a remote branch
• status PRs on current branch, PRs created by you. Assigned to you (requesting code review), CI status to be promoted to a top level command
• diff -- view a PRs changes really part of checkout
• show|info|get show PR, CI status, details, WIP status, open comments, unresolved issues.
• open|view|browse in browser
• list PRs associated to a ticket, or all of them, or by various other filters.
• assign -- assign to a user -- do with #54 (closed)
• ready| wip turn off WIP / toggle WIP

Smaller implementations

• close -- close without approving/merging
• ??reopen
• lock
• unlock

Implementations needing upstream commits to gitlab crate.

• approve
• merge (with rebase if needed?)