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.

mercurial/revset.py

@@ -571,6 +571,14 b' def _descendants(repo, subset, x, follow'
     if not args:
         return []
     s = set(_revdescendants(repo, args, followfirst)) | set(args)
+
+    if len(subset) == len(repo):
+        # the passed in revisions may not exist, -1 for example
+        for arg in args:
+            if arg not in subset:
+                s.remove(arg)
+        return list(s)
+
     return [r for r in subset if r in s]
 
 def descendants(repo, subset, x):
@@ -1328,6 +1336,9 b' def roots(repo, subset, x):'
     Changesets in set with no parent changeset in set.
     """
     s = set(getset(repo, repo.changelog, x))
+    if len(subset) == len(repo):
+        subset = s
+    else:
     subset = [r for r in subset if r in s]
     cs = _children(repo, subset, s)
     return [r for r in subset if r not in cs]