feat: Add support for merge request from fork

[Clement-Hue]

Summary

Add support for choosing and reviewing merge requests originating from forked projects. Previously, the plugin assumed the MR source branch always existed in the local repository, which broke fork-based workflows. This MR makes fork MRs discoverable, fetchable, and reviewable via choose_merge_request.

Problem

When a user selects a merge request that originates from a fork, the plugin fails because:

1. The MR middleware always filters by SourceBranch, which doesn't exist locally for fork MRs.
2. The pipeline handler cannot resolve the latest commit because the fork branch isn't on the remote.
3. Diffview can't display diffs because the fork commits aren't fetched.
4. The Lua choose_merge_request action tries to switch to a branch that doesn't exist locally.

Changes

• cmd/app/git/git.go — Add FetchMRHead (fetches refs/merge-requests/<iid>/head) and GetMRHeadCommit (resolves the fetched ref to a SHA) to the GitManager interface and Git implementation; add FetchMrHead helper that skips the fetch when no MR IID is set.
• cmd/app/git/git_test.go — Add unit tests for FetchMrHead (skip/fetch/error paths) and integration tests for GetMRHeadCommit (success and non-existent ref) using a temporary git repo.
• cmd/app/middleware.go — When ChosenMrIID is set, filter MRs by IID only (skip SourceBranch) so fork MRs are found.
• cmd/app/middleware_test.go — Add tests verifying the middleware uses IIDs vs SourceBranch depending on ChosenMrIID.
• cmd/app/pipeline.go — Fall back to GetMRHeadCommit when the branch doesn't exist on the remote and an MR IID is set.
• cmd/app/test_helpers.go — Implement GetMRHeadCommit and FetchMRHead on the FakeGitManager to satisfy the updated interface.
• cmd/main.go — Call FetchMrHead at startup so fork MR commits are available locally for diffview.
• lua/gitlab/actions/merge_requests.lua — Detect fork MRs (source_project_id ≠ target_project_id) and skip the branch switch; extract activate_mr, select_fork_mr, and select_local_mr helpers for clarity.
• lua/gitlab/hunks.lua — pass head_sha to parse_hunks_and_diff. The function now accepts and forwards head_sha to the git diff command, ensuring the diff is computed between the correct base and head SHAs of
  the merge request rather than comparing against the working tree. This make diff working with the new strategy

Suggestion

Probably it will be a good idea to use this approach also for local merge request and avoid the fetching of branch and checkout to it. This approach has several benefits

• No branch switch needed — your working tree stays untouched
• No clean tree requirement — you can have uncommitted changes
• Faster — no git switch, no buffer reloads
• Review any MR from any branch — you don't need the branch locally at all

[jakubbortlik]

Hi @Clement-Hue, I'm glad to see more contributors to the plugin. Just a note regarding your suggestion to use your new approach also for local merge requests. The benefit of using the local files for a review are that you can use all the benefits of LSP, code navigation, linting, etc. As for the perceived benefits of your approach, you can achieve at least some of them by using git worktrees. With worktrees you can have several versions of the same repo checked out at the same time and you don't worry about switching branches, stashing changes, and a clean tree. I find this a superior workflow for multiple feature branches regardless of merge requests.

The only reason I'm missing the possibility to review a MR without a local branch is for a closed MR where the feature branch has already been deleted.

I think it would be great if gitlab.nvim could support both these workflows.