git: allow setting symbolic ref for repos without master branch.

fixes  #5663

vcsserver/git.py

@@ -595,7 +595,7 b' class GitRemote(RemoteBase):'
 
             if refs and not update_after:
                 # mikhail: explicitly set the head to the last ref.
-                repo['HEAD'] = remote_refs[refs[-1]]
+                repo["HEAD"] = remote_refs[refs[-1]]
 
         if update_after:
             # we want to checkout HEAD
@@ -1203,6 +1203,13 b' class GitRemote(RemoteBase):'
         return install_git_hooks(path, bare, force_create=force)
 
     @reraise_safe_exceptions
+    def set_head_ref(self, wire, head_name):
+        log.debug('Setting refs/head to `%s`', head_name)
+        cmd = ['symbolic-ref', 'HEAD', 'refs/heads/%s' % head_name]
+        output, __ = self.run_git_command(wire, cmd)
+        return [head_name] + output.splitlines()
+
+    @reraise_safe_exceptions
     def get_hooks_info(self, wire):
         from vcsserver.hook_utils import (
             get_git_pre_hook_version, get_git_post_hook_version)