Fix appending trailers to git commits

Previously trailers were not properly added to end of git commit
messages. Apparently, trailers were overwriting the entire message
because the file was not opened and flushed properly.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>

src/git.py

@@ -121,8 +121,9 @@ def _set_trailers(self, patch: Patch) -> str:
         # Add trailers, changing them if they exist.
         # It's _highly_ unlikely that they'd exist, but this seems to be the
         # most sane way of handling that edge case.
-        with tempfile.NamedTemporaryFile(mode='wt') as f:
+        with tempfile.NamedTemporaryFile(mode='w+') as f:
           f.write(original_message)
+          f.flush() # Flush before we write to the file externally below
           self._git('interpret-trailers', '--in-place', '--if-exists=addIfDifferent',
                     '--trailer', change_id_trailer,  '--trailer', lore_trailer, f.name)
           return self._git.commit('--amend', '-F', f.name)