Fetch before Merge and push the squash commit to origin

internal/git/git.go

@@ -147,7 +147,7 @@ func UnmergedChanges(worktree, target string) (Unmerged, error) {
 	return u, nil
 }
 
-func SquashMerge(repo, worktree, target, message string, env []string) (string, error) {
+func SquashCommit(repo, worktree, target, message string, env []string) (string, error) {
 	checkedOut, err := run(repo, "symbolic-ref", "--short", "HEAD")
 	if err != nil {
 		return "", err
@@ -196,16 +196,40 @@ func SquashMerge(repo, worktree, target, message string, env []string) (string,
 		return "", nil
 	}
 
-	commit, err := runEnv(repo, env, "commit-tree", lines[0], "-p", head, "-m", message)
-	if err != nil {
-		return "", err
-	}
+	return runEnv(repo, env, "commit-tree", lines[0], "-p", head, "-m", message)
+}
 
-	_, err = run(repo, "merge", "--ff-only", commit)
+const Remote = "origin"
+
+var ErrRemoteMoved = errors.New("the remote's branch has moved")
+
+func Push(repo, commit, branch string) error {
+	out, stderr, code, err := runCode(repo, nil, "push", "--porcelain", Remote, commit+":refs/heads/"+branch)
 	if err != nil {
-		return "", err
+		return err
 	}
-	return commit, nil
+	if code == 0 {
+		return nil
+	}
+	for _, line := range strings.Split(out, "\n") {
+		if strings.HasPrefix(line, "!") && strings.Contains(line, "[rejected]") && (strings.Contains(line, "(fetch first)") || strings.Contains(line, "(non-fast-forward)")) {
+			return fmt.Errorf("pushing %s to %s: %w", branch, Remote, ErrRemoteMoved)
+		}
+	}
+	return fmt.Errorf("pushing %s to %s: exit status %d: %s", branch, Remote, code, strings.TrimSpace(out+"\n"+stderr))
+}
+
+func FastForward(repo, rev string) error {
+	_, err := run(repo, "merge", "--ff-only", rev)
+	return err
+}
+
+func FetchFastForward(repo, branch string) error {
+	_, err := run(repo, "fetch", Remote, branch)
+	if err != nil {
+		return err
+	}
+	return FastForward(repo, Remote+"/"+branch)
 }
 
 func Head(dir string) (string, error) {