patman: Support multi-line changes in changelogs

This patch adds support to multi-line changes. That is, if one has a line
in a changelog like
- Do a thing but
  it spans multiple lines
Using Series-process-log sort would sort as if those lines were unrelated.
With this patch, any change line starting with whitespace will be
considered part of the change before it.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>