handle "fugitive:.*" files in s:repl.includes_file

When sending code to the repl from a fugitive :Gd
or :Ge, remove the "fugitive://" prefix, and
"/.git/<rev number>/" path from the buffer name.

This allows for evaluation of code from previous
revisions of a file when inside of a fugitive :Gdiff
or :Gedit window.
Merge pull request #71 from rasmusto/fugitive-integration

handle "fugitive:.*" file in s:repl.includes_file