patman: Read in the git-mailrc alias file

We should read this file to obtain a set of aliases. This reduces the need
to create them in the ~/.patman file.

This feature did exist in some version of patman, and is mentioned in the
help but it did not find its way upstream.

Reported-by: Graeme Russ <gruss@tss-engineering.com>
Signed-off-by: Simon Glass <sjg@chromium.org>