Fix wt sync warning when upstream tracking ref is gone

e2e/testdata/sync.txtar

@@ -0,0 +1,51 @@
+[!exec:git] skip 'git not available'
+
+# Set up a project with a remote that has master + develop branches.
+setup-repo develop
+setup-project
+
+cd $WORK/project
+
+# Create worktrees for the existing local branches and configure upstream
+# tracking explicitly. Using git directly (not `wt add`) keeps the test focused
+# on sync behavior with guaranteed upstream setup.
+exec git --git-dir=.bare worktree add --relative-paths worktrees/master master
+exec git --git-dir=.bare worktree add --relative-paths worktrees/develop develop
+exec git -C worktrees/master branch --set-upstream-to=origin/master
+exec git -C worktrees/develop branch --set-upstream-to=origin/develop
+
+# Both upstreams resolve cleanly — sync reports up to date with no warnings.
+exec wt sync
+stderr 'master: up to date'
+stderr 'develop: up to date'
+! stderr 'could not check upstream'
+stderr 'Sync complete: 0 updated, 0 skipped, 0 failed'
+
+# Regression test: configured-but-gone upstream. When branch.<name>.merge points
+# at a non-existent remote ref, git's @{upstream} resolution fails with
+# "fatal: no such branch: 'HEAD..'". wt sync should treat this as up-to-date
+# and not surface a warning.
+exec git --git-dir=.bare config branch.develop.merge refs/heads/does-not-exist
+exec wt sync
+stderr 'develop: up to date'
+! stderr 'could not check upstream'
+! stderr 'no such branch'
+
+# Restore develop's upstream, push a new commit to remote develop, and verify
+# wt sync detects the lag and pulls.
+exec git --git-dir=.bare config branch.develop.merge refs/heads/develop
+
+cd $WORK/remote
+exec git checkout develop
+cp $WORK/newfile.txt newfile.txt
+exec git add newfile.txt
+exec git commit -m 'add newfile'
+
+cd $WORK/project
+exec wt sync
+stderr 'develop: pulling 1 commit'
+stderr 'Sync complete: 1 updated'
+exists worktrees/develop/newfile.txt
+
+-- newfile.txt --
+hello

internal/git/git.go

@@ -361,26 +361,31 @@ func (r *Runner) GetLastCommitAge(ctx context.Context, worktreePath string) (str
 }
 
 func (r *Runner) GetBehindCount(ctx context.Context, worktreePath string) (int, error) {
-	args := []string{"-C", worktreePath, "rev-list", "--count", "HEAD..@{upstream}"}
-	cmdStr := "git " + strings.Join(args, " ")
+	checkArgs := []string{"-C", worktreePath, "rev-parse", "--verify", "--quiet", "@{upstream}"}
+	checkStr := "git " + strings.Join(checkArgs, " ")
 
 	if r.DryRun {
-		ui.DryRunNotice(cmdStr)
+		ui.DryRunNotice(checkStr)
+		ui.DryRunNotice("git -C " + worktreePath + " rev-list --count HEAD..@{upstream}")
+		return 0, nil
+	}
+
+	ui.Command(checkStr)
+	if err := exec.CommandContext(ctx, "git", checkArgs...).Run(); err != nil {
 		return 0, nil
 	}
 
+	args := []string{"-C", worktreePath, "rev-list", "--count", "HEAD..@{upstream}"}
+	cmdStr := "git " + strings.Join(args, " ")
+
 	ui.Command(cmdStr)
 	cmd := exec.CommandContext(ctx, "git", args...)
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout = &stdout
 	cmd.Stderr = &stderr
 
 	if err := cmd.Run(); err != nil {
-		stderrStr := stderr.String()
-		if strings.Contains(stderrStr, "no upstream") || strings.Contains(stderrStr, "unknown revision") {
-			return 0, nil
-		}
-		return 0, fmt.Errorf("%s: %w\n%s", cmdStr, err, stderrStr)
+		return 0, fmt.Errorf("%s: %w\n%s", cmdStr, err, stderr.String())
 	}
 
 	return parseBehindCount(stdout.String()), nil