revset: improve head revset performance Previously the head() revset would iterate over every item in the subset and check if it was a head. Since the subset is often the entire repo, this was slow on large repos. Now we iterate over each item in the head list and check if it's in the subset, which results in much less work. hg log -r 'head()' on a large repo: Before: 0.95s After: 0.28s