[PATCH 3 of 3 V2] revset: cleanup of algorithm for `destination()` predicate

[Hannes Oldenburg]

# HG changeset patch
# User Hannes Oldenburg <hannes.christian.oldenburg at gmail.com>
# Date 1473552945 0
#      Sun Sep 11 00:15:45 2016 +0000
# Node ID 7eb48191ac8b5c679b8c606b2b83c0306180b97c
# Parent  05b4f301b5ade476f7f2236f7bf699a4f248297d
revset: cleanup of algorithm for `destination()` predicate

Previously the first `_getrevsource()` was treated as a special case, before
entering the loop and we kept track of two revs `r` and `src`, a target and
it's source respectively.

The argument for the correctness of the new approach goes like this:

We immediately add `r` to the lineage.
We update `r` to it's source, if it has no source `r` becomes None.
None is neither in `dests` nor in `sources`, so the lineage is thrown away
and the contents do not matter.
If however at some point, while following the sources, `r` is in `sources` or
`dests`, the lineage contains every preceding target and is added to `dests`.

diff -r 05b4f301b5ad -r 7eb48191ac8b mercurial/revset.py
--- a/mercurial/revset.py	Fri Sep 09 20:57:37 2016 +0000
+++ b/mercurial/revset.py	Sun Sep 11 00:15:45 2016 +0000
@@ -851,23 +851,20 @@
     # transitive transplants and rebases to yield the same results as transitive
     # grafts.
     for r in subset:
-        src = _getrevsource(repo, r)
         lineage = list()
         found = False
-        while src is not None and not found:
+        while r is not None and not found:
             lineage.append(r)
-
+            r = _getrevsource(repo, r)
             # The visited lineage is a match if the current source is in the arg
             # set.  Since every candidate dest is visited by way of iterating
             # subset, any dests further back in the lineage will be tested by a
             # different iteration over subset.  Likewise, if the src was already
             # selected, the current lineage can be selected without going back
             # further.
-            found = src in sources or src in dests
-            r = src
-            src = _getrevsource(repo, r)
+            found = r in sources or r in dests
         if found:
-            dests.update(linage)
+            dests.update(lineage)
     return subset.filter(dests.__contains__,
                          condrepr=lambda: '<destination %r>' % sorted(dests))