Add ,m keybinding to open master version in vsplit

<leader>m copies the current file name into register a, opens a new
vsplit, uses :r to read in from the command git show master:<filename>
and then turns on syntax highlighting by setting the filetype to Erlang.

I probably wouldn't have done this if not for "Learn Vimscript the Hard
Way", which encouraged me to set up my <leader>ev and <leader>sv
keybinds for editing and sourcing my .vimrc.after file (I'm using
Janus).