Additional changelog entries are modified when merging another branch

While trying to merge multiple branches into the mysql-dfsg-5.1 lucid branch I've found that starting from the second branch to merge debian/changelog had more modification that expected.

To reproduce:

Get the mysql-dfsg-5.1 lucid packaging branch:

  bzr init-repo mysql-pkg
  cd mysql-pkg/

  bzr co lp:ubuntu/lucid/mysql-dfsg-5.1 l

Get the two branches to merge into the packaging branch:
  bzr co lp:~mathiaz/ubuntu/lucid/mysql-dfsg-5.1/zap-bug-394515
  bzr co lp:~mathiaz/ubuntu/lucid/mysql-dfsg-5.1/zap-bug-444349

Create a local branch to work on the merge:
  bzr branch l l-bug-zap
  cd l-bug-zap/

Merge the first branch:
  bzr merge ../zap-bug-394515
  debcommit

Merge the second branch:
   bzr merge ../zap-bug-444349/

A conflict is created (normal):
  N debian/mysql-common.preinst
  M debian/changelog
  Text conflict in debian/changelog
  1 conflicts encountered.

Looking at the diff of debian/changelog:

   bzr diff debian/changelog
=== modified file 'debian/changelog'
--- debian/changelog 2010-03-31 23:37:09 +0000
+++ debian/changelog 2010-04-01 00:35:01 +0000
@@ -1,3 +1,4 @@
+<<<<<<< TREE
 mysql-dfsg-5.1 (5.1.41-3ubuntu10) UNRELEASED; urgency=low

   * debian/mysql-server-5.1.templates: fix MySQL Cluster package name to
@@ -5,6 +6,16 @@

  -- Mathias Gug <email address hidden> Wed, 31 Mar 2010 19:35:34 -0400

+=======
+mysql-dfsg-5.1 (5.1.41-3ubuntu10) UNRELEASED; urgency=low
+
+ * debian/mysql-common.preinst: delete skip-bdb option if upgrading from 5.0
+ since 5.1 fails to start if skip-bdb is in the configuration file.
+ (LP: #444349).
+
+ -- Mathias Gug <email address hidden> Wed, 31 Mar 2010 19:22:34 -0400
+
+>>>>>>> MERGE-SOURCE
 mysql-dfsg-5.1 (5.1.41-3ubuntu9) lucid; urgency=low

   * debian/{control, rules, mysql-testsuite.*}: Fix up testsuite package.
@@ -3234,9 +3245,15 @@

 mysql-dfsg (4.0.20-8) unstable; urgency=low

- * Upload stalled. Not released.
+ * The mysqlcheck that is started from the initscript will now be
+ backgrounded because it might else prevent the boot process to continue.
+ It also now notifies root by mail and syslog if a table is corrupt.
+ * The "debian-sys-maint" MySQL user now has almost full rights so that other
+ packages might use this account to create databases and user (thanks to
+ Andreas Barth). Closes: #262541
+ * Added paranoid rules for logcheck.

- -- Christian Hammers <email address hidden> Sun, 1 Aug 2004 20:27:55 +0200
+ -- Christian Hammers <email address hidden> Sun, 1 Aug 2004 21:00:55 +0200

 mysql-dfsg (4.0.20-7) unstable; urgency=medium

@@ -4758,6 +4775,14 @@

  -- Scott Hanson <email address hidden> Wed, 28 Jan 1998 18:59:09 +0100

+mysql (3.21.21.beta-1) unstable; urgency=low
+
+ * Use debhelper where possible in rules
+ * Split binary packages into mysql-base, mysql-client, mysql-doc
+ * New upstream release
+
+ -- Scott Hanson <email address hidden> Thu, 15 Jan 1998 08:12:17 +0100
+
 mysql (3.21.21a.beta-2) unstable; urgency=low

   * Compile with libpthreads from libc6-dev_2.0.6-3 rather than statically
@@ -4773,14 +4798,6 @@

  -- Scott Hanson <email address hidden> Mon, 19 Jan 1998 07:52:48 +0100

-mysql (3.21.21.beta-1) unstable; urgency=low
-
- * Use debhelper where possible in rules
- * Split binary packages into mysql-base, mysql-client, mysql-doc
- * New upstream release
-
- -- Scott Hanson <email address hidden> Thu, 15 Jan 1998 08:12:17 +0100
-
 mysql (3.21.19.beta-1) unstable; urgency=low

   * Offer to set root password in mysql_install_db

And compare that with the diff of the debian/changelog of branch to merge:

   cd ../zap-bug-444349
  bzr diff --old ../l/ debian/changelog
=== modified file 'debian/changelog'
--- debian/changelog 2010-03-25 09:08:05 +0000
+++ debian/changelog 2010-04-01 00:32:24 +0000
@@ -1,3 +1,11 @@
+mysql-dfsg-5.1 (5.1.41-3ubuntu10) UNRELEASED; urgency=low
+
+ * debian/mysql-common.preinst: delete skip-bdb option if upgrading from 5.0
+ since 5.1 fails to start if skip-bdb is in the configuration file.
+ (LP: #444349).
+
+ -- Mathias Gug <email address hidden> Wed, 31 Mar 2010 19:22:34 -0400
+
 mysql-dfsg-5.1 (5.1.41-3ubuntu9) lucid; urgency=low

   * debian/{control, rules, mysql-testsuite.*}: Fix up testsuite package.

There are additional modification in the debian/changelog (eg 3.21.21.beta-1).