revset: add optional offset argument to limit() predicate

It's common for GUI or web frontend to fetch chunk of revisions per batch
size. Previously it was possible only if revisions were sorted by revision
number.

  $ hg log -r 'limit({revspec} & :{last_known}, 101)'

So this patch introduces a general way to retrieve chunk of revisions after
skipping offset revisions.

  $ hg log -r 'limit({revspec}, 100, {last_count})'

This is a dumb implementation. We can optimize it for baseset and spanset
later.

mercurial/revset.py

@@ -1284,24 +1284,33 b' def keyword(repo, subset, x):'
     return subset.filter(matches)
 
 def limit(repo, subset, x):
-    """``limit(set, [n])``
-    First n members of set, defaulting to 1.
+    """``limit(set[, n[, offset]])``
+    First n members of set, defaulting to 1, starting from offset.
     """
-    args = getargsdict(x, 'limit', 'set n')
+    args = getargsdict(x, 'limit', 'set n offset')
     if 'set' not in args:
         # i18n: "limit" is a keyword
-        raise error.ParseError(_("limit requires one or two arguments"))
+        raise error.ParseError(_("limit requires one to three arguments"))
     try:
-        lim = 1
+        lim, ofs = 1, 0
         if 'n' in args:
             # i18n: "limit" is a keyword
             lim = int(getstring(args['n'], _("limit requires a number")))
+        if 'offset' in args:
+            # i18n: "limit" is a keyword
+            ofs = int(getstring(args['offset'], _("limit requires a number")))
+        if ofs < 0:
+            raise error.ParseError(_("negative offset"))
     except (TypeError, ValueError):
         # i18n: "limit" is a keyword
         raise error.ParseError(_("limit expects a number"))
     os = getset(repo, fullreposet(repo), args['set'])
     result = []
     it = iter(os)
+    for x in xrange(ofs):
+        y = next(it, None)
+        if y is None:
+            break
     for x in xrange(lim):
         y = next(it, None)
         if y is None:

tests/test-revset.t

@@ -566,6 +566,16 b' test ancestors'
   $ log 'keyword("test a")'
   $ log 'limit(head(), 1)'
   0
+  $ log 'limit(author("re:bob|test"), 3, 5)'
+  5
+  6
+  7
+  $ log 'limit(author("re:bob|test"), offset=6)'
+  6
+  $ log 'limit(author("re:bob|test"), offset=10)'
+  $ log 'limit(all(), 1, -1)'
+  hg: parse error: negative offset
+  [255]
   $ log 'matching(6)'
   6
   $ log 'matching(6:7, "phase parents user date branch summary files description substate")'