merge: fix --preview to show all nodes that will be merged (issue2043).

Formerly, it omitted nodes that were not descendants of the least
common ancestor of the two merge parents, even though those nodes
contribute to the merge.  The new algorithm uses revlog.findmissing()
instead of ancestor() + nodesbetween().

mercurial/commands.py

@@ -2233,14 +2233,14 b' def merge(ui, repo, node=None, **opts):'
         node = parent == bheads[0] and bheads[-1] or bheads[0]
 
     if opts.get('preview'):
-        p1 = repo['.']
-        p2 = repo[node]
-        common = p1.ancestor(p2)
-        roots, heads = [common.node()], [p2.node()]
+        # find nodes that are ancestors of p2 but not of p1
+        p1 = repo.lookup('.')
+        p2 = repo.lookup(node)
+        nodes = repo.changelog.findmissing(common=[p1], heads=[p2])
+
         displayer = cmdutil.show_changeset(ui, repo, opts)
-        for node in repo.changelog.nodesbetween(roots=roots, heads=heads)[0]:
-            if node not in roots:
-                displayer.show(repo[node])
+        for node in nodes:
+            displayer.show(repo[node])
         displayer.close()
         return 0
 

tests/test-merge-default

@@ -50,4 +50,11 b' hg commit -mf'
 echo % should fail because merge with other branch
 hg merge
 
+# Test for issue2043: ensure that 'merge -P' shows ancestors of 6 that
+# are not ancestors of 7, regardless of where their least common
+# ancestor is.
+echo % merge preview not affected by common ancestor
+hg up -q 7
+hg merge -q -P 6         # expect: 2, 4, 5, 6
+
 true

tests/test-merge-default.out

@@ -34,3 +34,8 b' created new head'
 % should fail because merge with other branch
 abort: branch 'foobranch' has one head - please merge with an explicit rev
 (run 'hg heads' to see all heads)
+% merge preview not affected by common ancestor
+2:2d95304fed5d
+4:f25cbe84d8b3
+5:a431fabd6039
+6:e88e33f3bf62