rebase: allow for rebasing descendants onto ancestors on different named branches

So far we've been denying rebasing descendants onto ancestors, but there are
situations in which this kind of operation makes perfect sense to me.

Let's say we have made a commit (or more), that belongs to branch 'dev', on
top of the named branch 'stable':

... a (stable) - b (dev)

but then we realize that b should belong to branch 'stable'.
In these cases a rebase means: "move these csets from named branch A to named
branch B" and there isn't a valid reason to deny it.

This patch basically doesn't block it, if source and destination are
on different named branches.
The old behaviour still applies for rebases across the same named branch.

Can you think of any tricky corner cases in which this new behaviour could
lead to problems? (I bet there are tons of them...)

By the way, I created a brand new .t because I feel there should be more
tests I can't think of at the moment.

tests/test-rebase-named-branches.t

@@ -0,0 +1,171 b''
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > graphlog=
+  > rebase=
+  > 
+  > [alias]
+  > tglog = log -G --template "{rev}: '{desc}' {branches}\n"
+  > EOF
+
+
+  $ hg init a
+  $ cd a
+
+  $ echo A > A
+  $ hg ci -Am A
+  adding A
+
+  $ echo B > B
+  $ hg ci -Am B
+  adding B
+
+  $ hg up -q -C 0
+
+  $ echo C > C
+  $ hg ci -Am C
+  adding C
+  created new head
+
+  $ hg up -q -C 0
+
+  $ echo D > D
+  $ hg ci -Am D
+  adding D
+  created new head
+
+  $ hg merge -r 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ hg ci -m E
+
+  $ hg up -q -C 3
+
+  $ echo F > F
+  $ hg ci -Am F
+  adding F
+  created new head
+
+  $ cd ..
+
+
+Rebasing descendant onto ancestor across different named branches
+
+  $ hg clone -q -u . a a1
+
+  $ cd a1
+
+  $ hg branch dev
+  marked working directory as branch dev
+
+  $ echo x > x
+
+  $ hg add x
+
+  $ hg ci -m 'extra named branch'
+
+  $ hg tglog
+  @  6: 'extra named branch' dev
+  |
+  o  5: 'F'
+  |
+  | o  4: 'E'
+  |/|
+  o |  3: 'D'
+  | |
+  | o  2: 'C'
+  |/
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+  $ hg rebase -s 6 -d 5
+  saved backup bundle to $TESTTMP/a1/.hg/strip-backup/*-backup.hg (glob)
+
+  $ hg tglog
+  @  6: 'extra named branch'
+  |
+  o  5: 'F'
+  |
+  | o  4: 'E'
+  |/|
+  o |  3: 'D'
+  | |
+  | o  2: 'C'
+  |/
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+  $ cd ..
+ 
+Rebasing descendant onto ancestor across the same named branches
+
+  $ hg clone -q -u . a a2
+
+  $ cd a2
+
+  $ echo x > x
+
+  $ hg add x
+
+  $ hg ci -m 'G'
+
+  $ hg tglog
+  @  6: 'G'
+  |
+  o  5: 'F'
+  |
+  | o  4: 'E'
+  |/|
+  o |  3: 'D'
+  | |
+  | o  2: 'C'
+  |/
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+  $ hg rebase -s 6 -d 5
+  abort: source is descendant of destination
+  [255]
+
+  $ cd ..
+ 
+Rebasing ancestor onto descendant across different named branches
+
+  $ hg clone -q -u . a a3
+
+  $ cd a3
+
+  $ hg branch dev
+  marked working directory as branch dev
+
+  $ echo x > x
+
+  $ hg add x
+
+  $ hg ci -m 'extra named branch'
+
+  $ hg tglog
+  @  6: 'extra named branch' dev
+  |
+  o  5: 'F'
+  |
+  | o  4: 'E'
+  |/|
+  o |  3: 'D'
+  | |
+  | o  2: 'C'
+  |/
+  | o  1: 'B'
+  |/
+  o  0: 'A'
+  
+  $ hg rebase -s 5 -d 6
+  abort: source is ancestor of destination
+  [255]
+
+  $ cd ..
+ 
+

hgext/rebase.py

@@ -482,9 +482,10 b' def buildstate(repo, dest, src, base, de'
 
     if src:
         commonbase = repo[src].ancestor(repo[dest])
+        samebranch = repo[src].branch() == repo[dest].branch()
         if commonbase == repo[src]:
             raise util.Abort(_('source is ancestor of destination'))
-        if commonbase == repo[dest]:
+        if samebranch and commonbase == repo[dest]:
             raise util.Abort(_('source is descendant of destination'))
         source = repo[src].rev()
         if detach: