buildman: Support upstream branch name containing /

Buildman assumes that branch names do not have a slash in them, since
slash is used to delimit remotes, etc. This means that a branch called
'WIP/tryme' in remote dm ends up being 'tryme'.

Adjust the logic a little, to try to accommodate this.

For now, no tests are added for this behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index b0a12f2..10ea5ff 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -147,8 +147,9 @@
     if remote == '.':
         return merge, None
     elif remote and merge:
-        leaf = merge.split('/')[-1]
-        return '%s/%s' % (remote, leaf), None
+        # Drop the initial refs/heads from merge
+        leaf = merge.split('/', maxsplit=2)[2:]
+        return '%s/%s' % (remote, '/'.join(leaf)), None
     else:
         raise ValueError("Cannot determine upstream branch for branch "
                          "'%s' remote='%s', merge='%s'"