feat!: implement AutoMerge option to AcceptMergeRequest

[jakubbortlik]

Closes #518.

This PR also fixes a bug when the Delete source branch setting of the MR was not respected due to incorrect reference in lua/gitlab/actions/merge.lua to state.INFO.delete_branch instead of state.INFO.force_remove_source_branch.

This PR also changes the behaviour of how the opts in gitlab.merge(opts) behave:

• Before this PR:

  • when lua require("gitlab").merge() was run without parameters, the values visible in the Summary view were used (except for the bug mentioned above).
  • specifying one of squash or delete_branch in opts caused the other parameter to default to false even if Summary view would show it's set to true.

• After this PR:

  • running lua require("gitlab").merge() without parameters doesn't change.
  • specifying any of the opts values (auto_merge, squash, delete_branch), has no effect on the other options - if not specified in the gitlab.merge() call, the values from the Summary are used.

This is a breaking change since before, a user could rely on the fact that ignoring one of the options would set it to false which now is not guaranteed - the value depends on the existing MR settings, which I find to be the desirable behaviour:

• only override what needs to be overridden
• only one set of default values (those in the Summary)

[jakubbortlik]

I'm thinking about adding this vim.ui.select call with a recursive call to M.merge, which would make it simpler for users to set auto-merge, but maybe it's a pattern that you (@harrisoncramer) don't want to introduce in the codebase.

  if state.INFO.detailed_merge_status ~= "mergeable" and not merge_body.auto_merge then
    vim.ui.select({ "Set auto-merge", "Cancel" }, {
      prompt = string.format(
        "MR not mergeable (currently '%s'). Set auto-merge instead?",
        state.INFO.detailed_merge_status
      ),
    }, function(choice)
      if choice == "Set auto-merge" then
        local opts_ = opts or {}
        opts_.auto_merge = true
        M.merge(opts_)  -- Recurse here with overridden `auto_merge`
      end
    end)
  else
    if merge_body.squash then
      local squash_message_popup = create_squash_message_popup()
      popup.set_up_autocommands(squash_message_popup, nil, vim.api.nvim_get_current_win())
      squash_message_popup:mount()
      popup.set_popup_keymaps(squash_message_popup, function(text)
        M.confirm_merge(merge_body, text)
      end, nil, popup.editable_popup_opts)
    else
      M.confirm_merge(merge_body)
    end
  end