copies: add test that makes both the merging csets dirty and fails

This patch is a part of series which is about the case when both the merging
csets are not descendant of merge base. The existing code assumes if c1 is
dirty there shouldn't be any partial copies from c2 i.e both2['incomplete']
and same for c2, if c2 is dirty both1['incomplete'] should be empty,
but this is not the right assumption.
Now as we know we can have both c1 and c2 dirty at the same time, it
is possible that c1 is dirty and both2['incomplete'] has some value.
Or if c2 is dirty and both1['incomplete'] has some value.

Added test shows that because of this assumption it could fail.

Differential Revision: https://phab.mercurial-scm.org/D5962

tests/test-copies.t

@@ -1,6 +1,8 b''
 #testcases filelog compatibility
 
   $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > rebase=
   > [alias]
   > l = log -G -T '{rev} {desc}\n{files}\n'
   > EOF
@@ -552,3 +554,80 b' Grafting revision 4 on top of revision 2'
       b
      +baba
   
+Test which demonstrate that fullcopytracing algorithm can fail to handle a case when both the csets are dirty
+----------------------------------------------------------------------------------------------------------
+
+  $ newrepo
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m "added a"
+  $ echo b > b
+  $ hg add b
+  $ hg ci -m "added b"
+
+  $ echo foobar > willconflict
+  $ hg add willconflict
+  $ hg ci -m "added willconflict"
+  $ echo c > c
+  $ hg add c
+  $ hg ci -m "added c"
+
+  $ hg l
+  @  3 added c
+  |  c
+  o  2 added willconflict
+  |  willconflict
+  o  1 added b
+  |  b
+  o  0 added a
+     a
+
+  $ hg up ".^^"
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ echo d > d
+  $ hg add d
+  $ hg ci -m "added d"
+  created new head
+
+  $ echo barfoo > willconflict
+  $ hg add willconflict
+  $ hg ci --amend -m "added willconflict and d"
+
+  $ hg l
+  @  5 added willconflict and d
+  |  d willconflict
+  | o  3 added c
+  | |  c
+  | o  2 added willconflict
+  |/   willconflict
+  o  1 added b
+  |  b
+  o  0 added a
+     a
+
+  $ hg rebase -r . -d 2 -t :other
+  rebasing 5:5018b1509e94 "added willconflict and d" (tip)
+
+  $ hg up 3 -q
+  $ hg l --hidden
+  o  6 added willconflict and d
+  |  d willconflict
+  | x  5 added willconflict and d
+  | |  d willconflict
+  | | x  4 added d
+  | |/   d
+  +---@  3 added c
+  | |    c
+  o |  2 added willconflict
+  |/   willconflict
+  o  1 added b
+  |  b
+  o  0 added a
+     a
+
+Now if we trigger a merge between cset revision 3 and 6 using base revision 4, in this case
+both the merging csets will be dirty as no one is descendent of base revision:
+
+  $ hg graft -r 6 --base 4 --hidden 2>&1 | grep "AssertionError"
+  AssertionError
+