patman: Use git to set up the test config

At present the branch information is dropped when writing the
configuration. It is easier to get git to set up the config anyway, so
update the test to do this.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/func_test.py

@@ -448,6 +448,11 @@ Changes in v2:
         self.repo = repo
         new_tree = repo.TreeBuilder().write()
 
+        common = ['git', f'--git-dir={self.gitdir}', 'config']
+        tools.run(*(common + ['user.name', 'Dummy']), cwd=self.gitdir)
+        tools.run(*(common + ['user.email', 'dumdum@dummy.com']),
+                  cwd=self.gitdir)
+
         # pylint doesn't seem to find this
         # pylint: disable=E1101
         author = pygit2.Signature('Test user', 'test@email.com')