revsets: preserve ordering with the or operator

This is valuable because now revsets like 'bookmarks() or tip' will
always show tip after bookmarks unless tip was itself a bookmark. This
is a somewhat contrived example, but this behavior is useful for
"where am I" type aliases that use log and revsets.

mercurial/revset.py

@@ -156,9 +156,10 b' def andset(repo, subset, x, y):'
     return getset(repo, getset(repo, subset, x), y)
 
 def orset(repo, subset, x, y):
-    s = set(getset(repo, subset, x))
-    s |= set(getset(repo, [r for r in subset if r not in s], y))
-    return [r for r in subset if r in s]
+    xl = getset(repo, subset, x)
+    s = set(xl)
+    yl = getset(repo, [r for r in subset if r not in s], y)
+    return xl + yl
 
 def notset(repo, subset, x):
     s = set(getset(repo, subset, x))

tests/test-revset.t

@@ -369,3 +369,8 b' issue2654: report a parse error if the r'
   hg: parse error at 2: invalid token
   [255]
 
+or operator should preserve ordering:
+  $ log 'reverse(2::4) or tip'
+  4
+  2
+  9