Don't crash for a new unsaved file
Don't crash for a new unsaved file

Fixes #53.

Test Plan:

 1. New file in a new tab with `cmd-n`
 2. Press `ctrl-b`
 3. Don't see an error

MichaelAquilina/git-blame,SlimeQ/git-graph,alexcorre/git-blame,crazymykl/git-blame,aotarola/git-blame