Support using a bare repo as wikidata

Right now, a bare Git repo will work fine for a read-only wiki, but writing will go through [`Data.FileStore.Git.gitSave`](https://github.com/jgm/filestore/blob/cb1425ef1d3935524ebc566c1bcfae5db1b978c7/Data/FileStore/Git.hs#L109-L115). At best this doesn't work (`"UnknownError: Could not git add 'foo' fatal: this operation must be run in a work tree "`), but if a file conflicted with the name of a file in a bare repo, it would simply be overridden.

A bare repo is more convenient if some users will be `git push`ing to the repo rather than using the web UI, so it'd be nice to see them supported. If you have a suggested technical approach (e.g. make filestore use libgit2 instead of shelling out to Git), I might be able to put some time towards this.