revpair: handle odd ranges (issue3474)

mercurial/scmutil.py

@@ -524,9 +524,11 b' def revpair(repo, revs):'
     l = revrange(repo, revs)
 
     if len(l) == 0:
+        if revs:
+            raise util.Abort(_('empty revision range'))
         return repo.dirstate.p1(), None
 
-    if len(l) == 1:
+    if len(l) == 1 and len(revs) == 1 and _revrangesep not in revs[0]:
         return repo.lookup(l[0]), None
 
     return repo.lookup(l[0]), repo.lookup(l[-1])

tests/test-diff-change.t

@@ -29,6 +29,12 b' Testing diff --change'
   -first
   +second
 
+Test dumb revspecs (issue3474)
+
+  $ hg diff -r 2:2
+  $ hg diff -r "2 and 1"
+  abort: empty revision range
+  [255]
 
 Testing diff --change when merge: