feat: update from origin/main without altering local branches

[guplem]

Summary

Follow-up to #111 as suggested by @pol-rivero.

After PR #111 added a fetch before the merge, performFetch also fast-forwards local branches as a side effect.
By merging origin/main directly (instead of local main) and skipping the fast-forward step, we avoid unintended changes to local branches.

So, instead of merging the local default branch (e.g. main), with this PR it merges the remote tracking branch (e.g. origin/main) directly. This means the "Update from Default Branch" action no longer alters local branches as a side effect. The menu label now reflects this (e.g. "Update from origin/main").

Changes

app/src/ui/app.tsx

• Made updateBranchWithContributionTargetBranch() async so it can await the fetch before merging.
• After fetching, resolves the remote tracking branch (e.g. origin/main) from allBranches and merges that instead of the local branch. Falls back to the local branch if no remote tracking branch is found.
• Added getRemoteTrackingBranch() helper: for remote branches (fork case) returns the branch as-is; for local branches, looks up the corresponding remote branch via the upstream field.

app/src/ui/dispatcher/dispatcher.ts

• Added fetchWithoutFastForward(): exposes the new app-store method to the UI layer.

app/src/lib/stores/app-store.ts

• Added skipFastForward parameter to performFetch(). When true, skips the fastForwardBranches() call so local branches are not altered.
• Added _fetchWithoutFastForward() public method that calls performFetch with skipFastForward: true.
• Updated the menu label source to use the branch's upstream name (e.g. origin/main) instead of the local name (e.g. main), so the menu reads "Update from origin/main".

Test plan

• Click Branch > Update from origin/main when the remote default branch has unpulled commits. Confirm the fetch runs and the merge succeeds.
• Click Branch > Update from origin/main when the branch is genuinely up to date. Confirm the app correctly reports "already up to date".
• Verify the local main branch is NOT fast-forwarded after the operation.
• Verify the menu label shows "Update from origin/main" (or the appropriate remote branch name).
• For fork repos contributing to parent: verify the menu shows "Update from upstream/main" and merges the upstream remote branch.

[pol-rivero]

This looks promising. I'll have a closer look later today

[pol-rivero]

I did some tests and this doesn't seem to be working...

This is what I did:

1. Go to main and make sure it's behind origin/main

2. Switch to a different branch (which is up-to-date with main but not origin/main)
3. Select "Branch" -> "Update from origin/main"
4. I would expect it to merge in the changes from origin/main, but instead it claims it's already up-to-date:

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In any case, you don't need to fix this problem, because I've been thinking about your original fix (fast-forwarding the main branch) and I now think that it was probably better than my proposal of merging the remote branch directly. I can't think of any case where fast-forwarding the main branch could be undesirable, and the diff of that PR is much smaller/cleaner than this one.
Sorry for making you waste time on this PR 😕

[guplem]

Uoops, I might have done something wrong, obviously… but yeah, I agree with you that this is probably not necessary. I was just trying to make everyone happy :)

So yeah, I am closing this PR then.

[pol-rivero]

Thanks for understanding ❤️