Concurrent git commands on the same repo fail

[mattwynne]

Related to #210, if you run a manual fetch immediately after a connect, you'll see a failure because the background fetch triggered by the domain rule seems to be clashing with the manual fetch.

The actual error I see is this:

[server]   <-- POST /repos/smoke-test-423955FC-68A3-4A72-B4ED-B9FCBEED12D8
[server] (node:7496) UnhandledPromiseRejectionWarning: Error: Git command `config gc.pruneExpire never` returned exit code 255:
[server] error: could not lock config file config: File exists
[server]
[server]     at GitDirectory.exec (/Users/matt/projects/git-en-boite/packages/local-git/src/git_directory.ts:27:13)
[server]     at processTicksAndRejections (internal/process/task_queues.js:97:5)
[server]     at exports.handleInit (/Users/matt/projects/git-en-boite/packages/local-git/src/handlers/handleInit.ts:9:3)
[server]     at RepoFactory.open (/Users/matt/projects/git-en-boite/packages/local-git/src/repo_factory.ts:49:5)
[server]     at Function.openGitRepo (/Users/matt/projects/git-en-boite/packages/local-git/src/dugite_git_repo.ts:17:22)
[server]     at BackgroundGitRepos.openGitRepo (/Users/matt/projects/git-en-boite/packages/local-git/src/background_git_repos.ts:64:21)
[server]     at DiskRepoIndex.find (/Users/matt/projects/git-en-boite/packages/repo-index/src/disk_repo_index.ts:20:29)
[server]     at LaBoîte.fetchFromRemote (/Users/matt/projects/git-en-boite/packages/app/src/la_boîte.ts:43:18)

So this looks like at the moment it's just caused the config commands we run in handleInit which we're blindly running each time you open a repo folder. We can probably avoid this, but there might be other problems with the other git commands when we get past this. Needs more investigation.