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.

Change-Id: I06fe0bfd47c008b3554d1c6cd06bd140997f3128

src/git.py

@@ -121,8 +121,10 @@ 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.seek(0, os.SEEK_END)
           f.write(original_message)
+          f.flush()
           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)