rebase: restrict rebase destination to the pulled set (issue5214)

Before this patch, `hg pull --rebase` would be a strict sequence of `hg pull`
followed by `hg rebase` if anything was pulled.

Now that rebase pick his default destination the same way than merge, than
`hg rebase` step would abort in the case the repo already had multiple anonymous
heads (because of the ambiguity). (changed in fac3a24be50e)

The intend of the user with `hg pull --rebase` is clearly to rebase on pulled
content. This used to be (mostly) enforced by the former default destination for
rebase, "tipmost changeset of the branch" as the tipmost would likely a
changeset that just got pulled. But this intended was no longer enforced with
the new defaul destination (unified with merge).

This changeset makes use of the '_destspace' mechanism introduced in the previous
changeset to enforce this.

This partially fixes issue5214 as no change at all have been made to the new
handling of the case with bookmark (unified with merge).

hgext/rebase.py

@@ -1244,6 +1244,9 def pullrebase(orig, ui, repo, *args, **
                 # --source.
                 if 'source' in opts:
                     del opts['source']
+                # revsprepull is the len of the repo, not revnum of tip.
+                destspace = list(repo.changelog.revs(start=revsprepull))
+                opts['_destspace'] = destspace
                 try:
                     rebase(ui, repo, **opts)
                 except error.NoMergeDestAbort:

tests/test-rebase-pull.t

@@ -311,3 +311,83 pull --rebase update (no rebase) use pro
   |
   o  0: 'C1'
   
+
+Multiple pre-existing heads on the branch
+-----------------------------------------
+
+Pull bring content, but nothing on the current branch, we should not consider
+pre-existing heads.
+
+  $ cd ../a
+  $ hg branch unrelatedbranch
+  marked working directory as branch unrelatedbranch
+  (branches are permanent and global, did you want a bookmark?)
+  $ echo B1 > B1
+  $ hg commit -Am B1
+  adding B1
+  $ cd ../c
+  $ hg up 'desc(L2)'
+  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg pull --rebase
+  pulling from $TESTTMP/a (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  nothing to rebase
+
+There is two local heads and we pull a third one.
+The second local head should not confuse the `hg pull rebase`.
+
+  $ hg up 'desc(R6)'
+  1 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ echo M1 > M1
+  $ hg commit -Am M1
+  adding M1
+  $ cd ../a
+  $ hg up 'desc(R6)'
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo R7 > R7
+  $ hg commit -Am R7
+  adding R7
+  $ cd ../c
+  $ hg up 'desc(L2)'
+  2 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ hg pull --rebase
+  pulling from $TESTTMP/a (glob)
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  rebasing 7:864e0a2d2614 "L1"
+  rebasing 8:6dc0ea5dcf55 "L2"
+  saved backup bundle to $TESTTMP/c/.hg/strip-backup/864e0a2d2614-2f72c89c-backup.hg (glob)
+  $ hg tglog
+  @  12: 'L2'
+  |
+  o  11: 'L1'
+  |
+  o  10: 'R7'
+  |
+  | o  9: 'M1'
+  |/
+  | o  8: 'B1' unrelatedbranch
+  |/
+  o  7: 'R6'
+  |
+  o  6: 'R5'
+  |
+  o  5: 'R4'
+  |
+  o  4: 'R3'
+  |
+  o  3: 'R2'
+  |
+  o  2: 'R1'
+  |
+  o  1: 'C2'
+  |
+  o  0: 'C1'
+