rebase: add a test showing that --dry-run wipes working copy changes

Eating people's data on --dry-run seems like a bad idea.

tests/test-rebase-dry-run.t

@@ -0,0 +1,99 b''
+======================
+rebase --dry-run tests
+======================
+
+Test behavior associated with `hg rebase --dry-run`
+
+Setup
+=====
+
+  $ hg init r1
+  $ cd r1
+  $ echo one > f01.txt
+  $ echo two > f02.txt
+  $ echo three > f03.txt
+  $ hg add
+  adding f01.txt
+  adding f02.txt
+  adding f03.txt
+  $ hg ci -m 'ci-1' f01.txt f02.txt f03.txt
+  $ hg book base; hg book -i
+  $ echo add-to-one >> f01.txt
+  $ hg ci -m 'br-1' f01.txt
+  $ hg book branch-1; hg book -i
+  $ hg up base; hg book -i
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (activating bookmark base)
+  $ echo add-to-two >> f02.txt
+  $ hg ci -m 'br-2' f02.txt
+  created new head
+  $ hg book branch-2; hg book -i
+  $ hg up branch-1; hg book -i
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (activating bookmark branch-1)
+  $ hg log -G
+  o  changeset:   2:d408211b0a6f
+  |  bookmark:    branch-2
+  |  tag:         tip
+  |  parent:      0:99418d161ee0
+  |  user:        test
+  |  date:        Thu Jan 01 00:00:00 1970 +0000
+  |  summary:     br-2
+  |
+  | @  changeset:   1:ab62441498e5
+  |/   bookmark:    branch-1
+  |    user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     br-1
+  |
+  o  changeset:   0:99418d161ee0
+     bookmark:    base
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     ci-1
+  
+
+
+Check the working copy changes do not get wiped out
+===================================================
+
+  $ echo add-to-three >> f03.txt
+
+f03 is modified
+
+  $ hg st
+  M f03.txt
+  $ hg diff
+  diff -r ab62441498e5 f03.txt
+  --- a/f03.txt	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/f03.txt	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,2 @@
+   three
+  +add-to-three
+
+
+  $ hg rebase -v -n -s branch-2 -d branch-1 --config extensions.rebase=
+  starting dry-run rebase; repository will not be changed
+  rebasing 2:d408211b0a6f branch-2 tip "br-2"
+  resolving manifests
+  getting f02.txt
+  committing files:
+  f02.txt
+  committing manifest
+  committing changelog
+  rebase merging completed
+  dry-run rebase completed successfully; run without -n/--dry-run to perform this rebase
+  resolving manifests (known-bad-output !)
+  getting f03.txt (known-bad-output !)
+
+f03 changes are lost
+
+  $ hg st
+  M f03.txt (missing-correct-output !)
+  $ hg diff
+  diff -r ab62441498e5 f03.txt (missing-correct-output !)
+  --- a/f03.txt	Thu Jan 01 00:00:00 1970 +0000 (missing-correct-output !)
+  +++ b/f03.txt	Thu Jan 01 00:00:00 1970 +0000 (missing-correct-output !)
+  @@ -1,1 +1,2 @@ (missing-correct-output !)
+   three (missing-correct-output !)
+  +add-to-three (missing-correct-output !)