Update mg_get_commit_section function

This function gets the initial commit message. It is based on
a fake commit message, done with `git -c commit.verbose=no commit`.
This is useful to catch any preparatory git message (template, hook, etc...).

Until now, the initial git commit message was fetched, and cleaned up of comment
in vimagit, mainly because of the '# Please enter the commit message ...'
postscript.

New method simply discard the postscript with `--no-status` option, and do not
touch the commit message with `--cleanup=verbatim`.

Options availaible at least since git v1.7.0

Fixes #196