merge-upstream

name	merge-upstream
description	Sync a fork branch with an upstream remote using a history-preserving merge. Use this whenever the user says /merge-upstream, merge upstream, sync upstream, sync fork, or wants upstream changes integrated without rebasing or force-pushing.

Merge Upstream

Sync the current fork branch with upstream/<branch> using a merge commit by default. Preserve local commit hashes, keep push non-destructive, and never rewrite history.

Usage

Treat these as equivalent triggers:

/merge-upstream [--base=<branch>] [--ff-allow]
merge upstream [--base=<branch>] [--ff-allow]
sync fork with upstream

Options:

• --base=<branch>: use that upstream branch instead of auto-detecting the upstream default branch.
• --ff-allow: allow git merge --ff-only when the current branch has no unique local commits. Without this, use git merge --no-ff to leave an explicit sync commit.

Invariants

• Do not run git rebase.
• Do not run git push --force or git push --force-with-lease.
• Do not run git stash push -a.
• Do not bypass hooks or signing with --no-verify or --no-gpg-sign.
• Ask before pushing.

Workflow

1. Validate the repository:

   git rev-parse --is-inside-work-tree
   git branch --show-current
   git remote get-url upstream
   git remote get-url origin
   

   Abort on detached HEAD or missing upstream. If origin is missing, continue locally and skip push.

2. If the worktree is dirty, auto-stash tracked and untracked files:

   git stash push -u -m "merge-upstream auto-stash $(date +%Y%m%d-%H%M%S)"
   git stash list -n 1 --format='%gd'
   git status --porcelain
   

   Track the stash ref. If the worktree is still dirty after stashing, stop and ask the user to clean it manually.

3. Detect the upstream target branch:

   • If --base=<branch> is provided, fetch and verify upstream/<branch>.
   • Otherwise run git remote set-head upstream -a, then read refs/remotes/upstream/HEAD.
   • If detection fails, ask the user for --base=<branch>.

4. Fetch enough history for a reliable merge base:

   git rev-parse --is-shallow-repository
   git fetch --tags upstream "+refs/heads/${upstream_branch}:refs/remotes/upstream/${upstream_branch}"
   git fetch origin "+refs/heads/${current_branch}:refs/remotes/origin/${current_branch}"
   git merge-base HEAD "upstream/${upstream_branch}"
   

   If the repository is shallow, unshallow origin first when available, then upstream only if still shallow.

5. Report divergence:

   git rev-list --count "upstream/${upstream_branch}..HEAD"
   git rev-list --count "HEAD..upstream/${upstream_branch}"
   GIT_PAGER=cat git log --oneline "HEAD..upstream/${upstream_branch}"
   GIT_PAGER=cat git log --first-parent --oneline "upstream/${upstream_branch}..HEAD"
   

6. Merge:

   • If behind is 0, skip merge.

   • If behind is greater than 0 and --ff-allow is set with ahead 0, run:

     git merge --ff-only "upstream/${upstream_branch}"
     

   • Otherwise capture previous_head and upstream_tip, then run:

     git merge --no-ff "upstream/${upstream_branch}" -m "merge: sync ${current_branch} with upstream/${upstream_branch}"
     

7. Resolve conflicts locally when they occur:

   git diff --name-only --diff-filter=U
   

   Read each conflicted file. Auto-resolve only mechanically obvious conflicts such as non-overlapping additions, import unions, formatting-only differences, or generated lockfile refreshes. For semantic conflicts, present the specific conflict and ask whether to keep ours, keep theirs, manually edit, or abort. After resolution:

   git add <file>
   git -c core.editor=true merge --continue
   

8. Verify:

   git rev-parse --git-path MERGE_HEAD
   git rev-parse --git-path rebase-merge
   git rev-parse --git-path rebase-apply
   git show -s --format=%P HEAD
   git rev-list --left-right --count "upstream/${upstream_branch}...HEAD"
   git merge-base --is-ancestor "origin/${current_branch}" HEAD
   

   In default mode, verify HEAD has two parents: previous_head as first parent and upstream_tip as second parent. In --ff-allow fast-forward mode, verify HEAD equals upstream_tip.

9. Push only with explicit approval:

   GIT_PAGER=cat git log --oneline --graph --decorate -10
   git push origin "${current_branch}"
   

   If the remote branch does not exist, use git push -u origin "${current_branch}". If push is rejected as non-fast-forward, re-fetch and offer only non-destructive options: merge origin/<branch> into HEAD and retry, or stop.

10. Restore the stash when one was created:

git stash pop "$stash_ref"

If pop conflicts, leave the stash entry in place and report the exact ref.

Final Report

Include:

• branch and upstream target
• whether merge, fast-forward, or no-op happened
• fork commits preserved
• upstream commits integrated
• push status
• stash status
• any conflicts and how they were resolved