diff: don't crash when merged-in addition was removed (issue4786)

During a merge, if the user removes a file that came from parent 2 and
did not exist in parent 1, the file's status will be "removed". This
surprises the diff code, which crashes because it expects removed
files exist in parent 1. This has been broken since 377124ba6b10
(trydiff: use 'not in addedset' for symmetry with 'not in removedset',
2014-12-23).

Fix by fixing up the list of removed file, similar to how we currently
fix up the list of modified and added files during a merge.

mercurial/patch.py

@@ -2258,14 +2258,21 b' def diff(repo, node1=None, node2=None, m'
 
     modifiedset = set(modified)
     addedset = set(added)
+    removedset = set(removed)
     for f in modified:
         if f not in ctx1:
             # Fix up added, since merged-in additions appear as
             # modifications during merges
             modifiedset.remove(f)
             addedset.add(f)
+    for f in removed:
+        if f not in ctx1:
+            # Merged-in additions that are then removed are reported as removed.
+            # They are not in ctx1, so We don't want to show them in the diff.
+            removedset.remove(f)
     modified = sorted(modifiedset)
     added = sorted(addedset)
+    removed = sorted(removedset)
 
     def difffn(opts, losedata):
         return trydiff(repo, revs, ctx1, ctx2, modified, added, removed,

tests/test-diffdir.t

@@ -38,3 +38,33 b''
   $ hg diff -r tip -r ""
   hg: parse error: empty query
   [255]
+
+Remove a file that was added via merge. Since the file is not in parent 1,
+it should not be in the diff.
+
+  $ hg ci -m 'a=foo' a
+  $ hg co -Cq null
+  $ echo 123 > b
+  $ hg add b
+  $ hg ci -m "b"
+  created new head
+  $ hg merge 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg rm -f a
+  $ hg diff --nodates
+
+Rename a file that was added via merge. Since the rename source is not in
+parent 1, the diff should be relative to /dev/null
+
+  $ hg co -Cq 2
+  $ hg merge 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg mv a a2
+  $ hg diff --nodates
+  diff -r cf44b38435e5 a2
+  --- /dev/null
+  +++ b/a2
+  @@ -0,0 +1,1 @@
+  +foo