cmdutil: changed code in _makegraphlogrevset not to use getitem

__getitem__ is a method that is not implemented lazily on many of the new
classes and it can be easily replaced with a structure that takes advantage of
the new lazy implementations instead.

mercurial/cmdutil.py

@@ -1515,8 +1515,12 b' def _makegraphlogrevset(repo, pats, opts'
     follow = opts.get('follow') or opts.get('follow_first')
     followfirst = opts.get('follow_first') and 1 or 0
     # --follow with FILE behaviour depends on revs...
-    startrev = revs[0]
-    followdescendants = (len(revs) > 1 and revs[0] < revs[1]) and 1 or 0
+    it = iter(revs)
+    startrev = it.next()
+    try:
+        followdescendants = startrev < it.next()
+    except (StopIteration):
+        followdescendants = False
 
     # branch and only_branch are really aliases and must be handled at
     # the same time