commit: increase perf by avoiding checks against entire repo subsets When commiting to a repo with lots of history (>400000 changesets) checking the results of revset.py:descendants against the subset takes some time. Since the subset equals the entire changelog, the check isn't necessary. Avoiding it in that case saves 0.1 seconds off of a 1.78 second commit. A 6% gain. We use the length of the subset to determine if it is the entire repo. There is precedence for this in revset.py:stringset.