ssoma-mda: Use the email subject as the commit message

ssoma-mda: Use the email subject as the commit message

This is more interesting than just using 'mda' all the time, but it's
harder to setup proper quoting around the message without using
third-party Perl modules (e.g. IPC::Run or String::ShellQuote).  This
proof-of-concept patch just assumes the subject doesn't contain
single-quotes (').  This patch also doesn't handle the empty/missing
subject case, which should probably fall back to '<no subject>' or
some such.

I'm fine dropping support for older Gits here, and just using the -m
option to commit-tree.  That landed with 96b8d93a (commit-tree: teach
-m/-F options to read logs from elsewhere, 2011-11-09) in Git v1.7.9,
which was released over 2.5 years ago on 2012-01-27.

It would also be useful (I think) to set the GIT_AUTHOR_NAME,
GIT_AUTHOR_EMAIL, and GIT_AUTHOR_DATE environment variables from the
message header before committing.  I know how to do that using
Python's subprocess module, but I don't know the Perl incantation.