Bare repositories don't work with git 2.25

[campersau]

It looks like git version 2.25 changed the behavior of git rev-parse --show-toplevel inside of bare repositories. It now throws an error: fatal: this operation must be run in a work tree

That's the reason why travis with the latest git version and all appveyor builds are failing. Appveyor updated the git version a few days ago.

#1263 (comment)