corruption: add a test for issue6528 stable

corruption: add a test for issue6528

The initial reproduction script was provided by Charles Chamberlain from Jane
Street.

Differential Revision: https://phab.mercurial-scm.org/D10996

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-issue6528.t	Tue Jul 06 19:22:50 2021 +0200
@@ -0,0 +1,171 @@
+===============================================================
+Test non-regression on the corruption associated with issue6528
+===============================================================
+
+Setup
+-----
+
+  $ hg init base-repo
+  $ cd base-repo
+
+  $ cat <<EOF > a.txt
+  > 1
+  > 2
+  > 3
+  > 4
+  > 5
+  > 6
+  > EOF
+
+  $ hg add a.txt
+  $ hg commit -m 'c_base_c - create a.txt'
+
+Modify a.txt
+
+  $ sed -e 's/1/foo/' a.txt > a.tmp; mv a.tmp a.txt
+  $ hg commit -m 'c_modify_c - modify a.txt'
+
+Modify and rename a.txt to b.txt
+
+  $ hg up -r "desc('c_base_c')"
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ sed -e 's/6/bar/' a.txt > a.tmp; mv a.tmp a.txt
+  $ hg mv a.txt b.txt
+  $ hg commit -m 'c_rename_c - rename and modify a.txt to b.txt'
+  created new head
+
+Merge each branch
+
+  $ hg merge -r "desc('c_modify_c')"
+  merging b.txt and a.txt to b.txt
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -m 'c_merge_c: commit merge'
+
+  $ hg debugrevlogindex b.txt
+     rev linkrev nodeid       p1           p2
+       0       2 05b806ebe5ea 000000000000 000000000000
+       1       3 a58b36ad6b65 000000000000 05b806ebe5ea
+
+Check commit Graph
+
+  $ hg log -G
+  @    changeset:   3:a1cc2bdca0aa
+  |\   tag:         tip
+  | |  parent:      2:615c6ccefd15
+  | |  parent:      1:373d507f4667
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     c_merge_c: commit merge
+  | |
+  | o  changeset:   2:615c6ccefd15
+  | |  parent:      0:f5a5a568022f
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     c_rename_c - rename and modify a.txt to b.txt
+  | |
+  o |  changeset:   1:373d507f4667
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     c_modify_c - modify a.txt
+  |
+  o  changeset:   0:f5a5a568022f
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     c_base_c - create a.txt
+  
+
+  $ hg cat -r . b.txt
+  foo
+  2
+  3
+  4
+  5
+  bar
+  $ cat b.txt
+  foo
+  2
+  3
+  4
+  5
+  bar
+  $ cd ..
+
+
+Check the lack of corruption
+----------------------------
+
+  $ hg clone --pull base-repo cloned
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 4 changesets with 4 changes to 2 files
+  new changesets f5a5a568022f:a1cc2bdca0aa
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd cloned
+  $ hg up -r "desc('c_merge_c')"
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+
+Status is buggy, even with debugrebuilddirstate
+
+  $ hg cat -r . b.txt
+  foo
+  2
+  3
+  4
+  5
+  bar
+  $ cat b.txt
+  foo
+  2
+  3
+  4
+  5
+  bar
+  $ hg status
+  M b.txt (known-bad-output !)
+  $ hg debugrebuilddirstate
+  $ hg status
+  M b.txt (known-bad-output !)
+
+the history was altered
+
+in theory p1/p2 order does not matter but in practice p1 == nullid is used as a
+marker that some metadata are present and should be fetched.
+
+  $ hg debugrevlogindex b.txt
+     rev linkrev nodeid       p1           p2
+       0       2 05b806ebe5ea 000000000000 000000000000
+       1       3 a58b36ad6b65 05b806ebe5ea 000000000000 (known-bad-output !)
+       1       3 a58b36ad6b65 000000000000 05b806ebe5ea (missing-correct-output !)
+
+Check commit Graph
+
+  $ hg log -G
+  @    changeset:   3:a1cc2bdca0aa
+  |\   tag:         tip
+  | |  parent:      2:615c6ccefd15
+  | |  parent:      1:373d507f4667
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     c_merge_c: commit merge
+  | |
+  | o  changeset:   2:615c6ccefd15
+  | |  parent:      0:f5a5a568022f
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     c_rename_c - rename and modify a.txt to b.txt
+  | |
+  o |  changeset:   1:373d507f4667
+  |/   user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     c_modify_c - modify a.txt
+  |
+  o  changeset:   0:f5a5a568022f
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     c_base_c - create a.txt
+