Copy a ref's name to the clipboard from its menu - #711

A branch name too long for the sidebar to show has to be read off the
screen and retyped before it can be used, so the menu that every ref
already has now offers to copy it.

Description

• Add the item to menuItemsForRef:, the one builder the sidebar and
  the ref labels in the history view both read, so both gain it at once
• Copy the short name, feature/foo, which is what a git command
  takes; the full ref would have to be trimmed by hand
• Reach the ref through refishForSender:refishTypes: with no type
  filter, so branches, tags, remotes and remote branches are all
  covered by the same action
• Put it second, directly under the entry that opens or checks the ref
  out, rather than beside the one that removes it, and title it
  "Copy name" so it is clear what lands on the clipboard

Screenshot

Test plan

• 183 unit tests pass, six covering this item: it is offered and
  enabled, carries its ref, names the branch rather than the full ref,
  is offered for tags and remote branches, sits second whether the
  first entry checks out or opens a worktree, comes first for a plain
  remote which has neither, and is never adjacent to Remove
• Checked by hand in a running build against the GitX repo: the menu
  for master reads Checkout “master” then Copy name “master”, and
  invoking it puts master on the clipboard, not refs/heads/master
• No automated test covers the pasteboard write itself. NSPasteboard generalPasteboard is system-wide, so a test would replace whatever
  the developer had copied, and restoring it would preserve only plain
  text. The wiring, the title, the placement and the ref types are
  covered instead.