handle the lack of LFS locking API on the remote repository

#2747 highlighted some Git scenarios that we don’t handle properly, and one interesting one was a push to an LFS-enabled repository that doesn’t support locking:

2017-09-21T01:19:40.407Z - error: [ui] `git -c credential.helper= push origin master:master --progress` exited with an unexpected code: 1.
Remote "origin" does not support the LFS locking API. Consider disabling it with:
  $ git config lfs.https://github.com/AndyBPA/brutes.git/info/lfs.locksverify false

This experience could be better:

• as part of initializing LFS for the repository, we might be able to probe for it and set it
• when the push fails, we could warn the user, set it for them and retry

Probably need to do both, but it’d be nice to get some feedback from @technoweenie or someone on the LFS team about what we should be doing here to lay the groundwork for locking support

[technoweenie]

@AndyBPA You can turn off the locks checks by running git config lfs.locksverify false. The lfs.<url>.locksverify config keys let you set this on a per-remote or per host basis.

[joshaber]

It takes a bit of work to assemble all the values GitHub Desktop needs. Here’s an example with my computer:

GIT_TRACE=1 DESKTOP_ASKPASS_SCRIPT="/Users/joshaber/Documents/Development/GitHub/desktop/dist/GitHub Desktop-dev-darwin-x64/GitHub Desktop-dev.app/Contents/Resources/app/ask-pass.js" DESKTOP_ENDPOINT="https://api.github.com" DESKTOP_PATH="/Users/joshaber/Documents/Development/GitHub/desktop/dist/GitHub Desktop-dev-darwin-x64/GitHub Desktop-dev.app/Contents/Frameworks/GitHub Desktop-dev Helper.app/Contents/MacOS/GitHub Desktop-dev Helper" DESKTOP_USERNAME="joshaber" GIT_ASKPASS="/Users/joshaber/Documents/Development/GitHub/desktop/dist/GitHub Desktop-dev-darwin-x64/GitHub Desktop-dev.app/Contents/Resources/app/static/ask-pass-trampoline.sh" git -c "credential.helper=" lfs fetch —all

There are a lot of paths there that will need to be changed to be relative to where the app is on a particular machine.

I’m gonna open a PR that adds the ability to have Desktop use GIT_TRACE.