Emacs: configure git-messenger package

This patch configures git-messenger package which shows the last
commit message pertaining to the line at cursor.

This is bound to C-x v p

Tested-by: Ratheesh S <ratheeshreddy@gmail.com>
Signed-off-by: Ratheesh S <ratheeshreddy@gmail.com>
Emacs: some more cosmetic changes to dark-theme

Signed-off-by: Ratheesh S <ratheeshreddy@gmail.com>