Add a newline between commit messages

Git commit message presubmit test might merge multiple commit messages
together if there is no newline in between.

Bug: angleproject:4923
Change-Id: I668b7d1f3c7e17d7499bf1fd2b9390d77ce70d5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2339754Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

git_cl.py

@@ -902,7 +902,7 @@ def _create_description_from_log(args):
     log_args = [args[0] + '..' + args[1]]
   else:
     log_args = args[:]  # Hope for the best!
-  return RunGit(['log', '--pretty=format:%B'] + log_args)
+  return RunGit(['log', '--pretty=format:%B%n'] + log_args)
 
 
 class GerritChangeNotExists(Exception):