graphmod: avoid sorting when already sorted

This is somewhat redundant now, but allows us to add a toposort that should not
be re-sorted either.

mercurial/cmdutil.py

@@ -2147,6 +2147,7 b' def getgraphlogrevs(repo, pats, opts):'
     if opts.get('rev'):
         # User-specified revs might be unsorted, but don't sort before
         # _makelogrevset because it might depend on the order of revs
+        if not revs.isdescending():
         revs.sort(reverse=True)
     if expr:
         # Revset matchers often operate faster on revisions in changelog