devtool: upgrade: fix constructing new branch from tarball releases

When we do an upgrade from one tarball version to another we want to:

1) Check out the old version as a new branch
2) Record the changes between the old and new versions as a commit
3) Check out the old version with patches applied
4) Rebase that onto the new branch

Where we went wrong was step #1 where instead we checked out the old
version with patches applied as the new branch, which meant the rebase
didn't do anything and any changes made by the patches to files still in
the new version were wiped out.

(From OE-Core rev: a4f3e6d5f4c549e36cce1e18801a565a3a3c9b0a)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/devtool/upgrade.py

@@ -179,7 +179,7 @@ def _extract_new_source(newpv, srctree, no_patch, srcrev, branch, keep_temp, tin
         md5 = None
         sha256 = None
     else:
-        __run('git checkout -b devtool-%s' % newpv)
+        __run('git checkout devtool-base -b devtool-%s' % newpv)
 
         tmpdir = tempfile.mkdtemp(prefix='devtool')
         try: