When sync-branch is configured, check that branch's upstream instead of
current HEAD's upstream. This fixes --auto-push with jj/jujutsu which
always operates in detached HEAD mode.
Adds gitBranchHasUpstream(branch) to check specific branch's upstream
tracking, independent of current HEAD state.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>