rebase: return early when source is descendant of destination

This only happens when using --base (or no source selection options), as
rebase already aborts in this situation when using --source.

Without this change you get an abort from the underlying merge, and the
repository is in a different state than you started with (the working
dir parent is changed).

hgext/rebase.py

@@ -401,6 +401,10 b' def buildstate(repo, dest, src, base, co'
             return None
 
         cwdancestors = set(repo.changelog.ancestors(cwd))
+        if dest in cwdancestors:
+            repo.ui.debug('source is descendant of destination\n')
+            return None
+
         cwdancestors.add(cwd)
         rebasingbranch = cwdancestors - targetancestors
         source = min(rebasingbranch)

tests/test-rebase-scenario-global

@@ -70,7 +70,7 b" echo '% D onto B - E maintains C as pare"
 hg rebase -s 3 -d 1 2>&1 | sed 's/\(saving bundle to \).*/\1/'
 hg glog  --template '{rev}: {desc}\n'
 
-echo '% These will fail'
+echo '% These will fail (using --source)'
 createrepo > /dev/null 2>&1
 echo '% E onto D - rebase onto an ancestor'
 hg rebase -s 4 -d 3
@@ -79,4 +79,13 b' hg rebase -s 3 -d 4'
 echo '% E onto B - merge revision with both parents not in ancestors of target'
 hg rebase -s 4 -d 1
 
+echo
+echo '% These will abort gracefully (using --base)'
+echo '% E onto E - rebase onto same changeset'
+hg rebase -b 4 -d 4
+echo '% E onto D - rebase onto an ancestor'
+hg rebase -b 4 -d 3
+echo '% D onto E - rebase onto a descendant'
+hg rebase -b 3 -d 4
+
 exit 0

tests/test-rebase-scenario-global.out

@@ -127,10 +127,18 b' o | 1: B'
 |/
 o  0: A
 
-% These will fail
+% These will fail (using --source)
 % E onto D - rebase onto an ancestor
 abort: source is descendant of destination
 % D onto E - rebase onto a descendant
 abort: source is ancestor of destination
 % E onto B - merge revision with both parents not in ancestors of target
 abort: cannot use revision 4 as base, result would have 3 parents
+
+% These will abort gracefully (using --base)
+% E onto E - rebase onto same changeset
+nothing to rebase
+% E onto D - rebase onto an ancestor
+nothing to rebase
+% D onto E - rebase onto a descendant
+nothing to rebase