revset: remove existence check from min() and max()

revset: remove existence check from min() and max()

min() and max() would first do an existence check. Unfortunately existence
checks can be slow in certain situations (like if the smartset is a list, and
quickly iterable in both ascending and descending directions, then doing an
existence check will start from the bottom, even if you want to check the
max()).

The fix is to not do the check, and just handle the error if it happens. In a
large repo, this speeds up:

hg log -r 'max(parents(. + .^) - (. + .^)  & ::master)'

from 3.5s to 0.85s. That revset is contrived and just for testing. In our
real case we used 'bundle()' in place of '. + .^'

Interesting perf numbers for the revset benchmarks:

max(draft() and ::tip) =>  0.027s to 0.0005s
max(author(lmoscovicz)) => 2.48s to 0.57s

min doesn't show any perf changes, but changing it as well will prevent a perf
regression in my next patch.

Result from revset benchmark

revset #0: draft() and ::tip
   min           max
0) 0.001971      0.001991
1) 0.001965      0.000428  21%

revset #1: ::tip and draft()
   min           max
0) 0.002017      0.001912
1) 0.001896  94% 0.000421  22%

revset #2: author(lmoscovicz)
   min           max
0) 1.049033      1.358913
1) 1.042508      0.319824  23%

revset #3: author(lmoscovicz) or author(mpm)
   min           max
0) 1.042512      1.367432
1) 1.019750      0.327750  23%

revset #4: author(mpm) or author(lmoscovicz)
   min           max
0) 1.050135      0.324924
1) 1.070698      0.319913

revset #5: roots((tip~100::) - (tip~100::tip))
   min           max
0) 0.000671      0.001018
1) 0.000605  90% 0.000946  92%

revset #6: roots((0::) - (0::tip))
   min           max
0) 0.149714      0.152369
1) 0.098677  65% 0.100374  65%

revset #7: (20000::) - (20000)
   min           max
0) 0.051019      0.042747
1) 0.035586  69% 0.016267  38%

--- a/mercurial/revset.py	Fri Sep 18 17:23:10 2015 -0700
+++ b/mercurial/revset.py	Sun Sep 20 19:27:53 2015 -0700
@@ -1387,10 +1387,14 @@
     Changeset with highest revision number in set.
     """
     os = getset(repo, fullreposet(repo), x)
-    if os:
+    try:
         m = os.max()
         if m in subset:
             return baseset([m])
+    except ValueError:
+        # os.max() throws a ValueError when the collection is empty.
+        # Same as python's max().
+        pass
     return baseset()
 
 def merge(repo, subset, x):
@@ -1426,10 +1430,14 @@
     Changeset with lowest revision number in set.
     """
     os = getset(repo, fullreposet(repo), x)
-    if os:
+    try:
         m = os.min()
         if m in subset:
             return baseset([m])
+    except ValueError:
+        # os.min() throws a ValueError when the collection is empty.
+        # Same as python's min().
+        pass
     return baseset()
 
 def modifies(repo, subset, x):