Do not abort rollback if undo.branch isn't available, but warn.

mercurial/localrepo.py

@@ -579,8 +579,13 b' class localrepository(repo.repository):'
                 self.ui.status(_("rolling back last transaction\n"))
                 transaction.rollback(self.sopener, self.sjoin("undo"))
                 util.rename(self.join("undo.dirstate"), self.join("dirstate"))
-                branch = self.opener("undo.branch").read()
-                self.dirstate.setbranch(branch)
+                try:
+                    branch = self.opener("undo.branch").read()
+                    self.dirstate.setbranch(branch)
+                except IOError:
+                    self.ui.warn(_("Named branch could not be reset, "
+                                   "current branch still is: %s\n")
+                                 % util.tolocal(self.dirstate.branch()))
                 self.invalidate()
                 self.dirstate.invalidate()
             else:

tests/test-rollback

@@ -14,8 +14,15 b' hg verify'
 hg parents
 hg status
 
-# Test issue 902
+echo % Test issue 902
 hg commit -m "test"
 hg branch test
 hg rollback
 hg branch
+
+echo % Test rollback of hg before issue 902 was fixed
+hg commit -m "test"
+hg branch test
+rm .hg/undo.branch
+hg rollback
+hg branch

tests/test-rollback.out

@@ -16,6 +16,12 b' crosschecking files in changesets and ma'
 checking files
 0 files, 0 changesets, 0 total revisions
 A a
+% Test issue 902
 marked working directory as branch test
 rolling back last transaction
 default
+% Test rollback of hg before issue 902 was fixed
+marked working directory as branch test
+rolling back last transaction
+Named branch could not be reset, current branch still is: test
+test