automate updating the embedded Git details

[shiftkey]

Following on from desktop/dugite-native#115, this PR makes it a one-liner to update the embedded Git in dugite to the latest release of dugite-native.

npm run update-embedded-git

This required moving a bit of the internals around:

• introduce a script/embedded-git.json JSON object that we use when installing or updating dugite, rather than our previous hard-coded logic
• use a consistent ${process.platform}-${os.arch()} convention for resolving versions (like before, if we don't have a version that matches here, we skip the download step).

And because the Git and Git LFS versions have updated recently, we've updated the test/helpers.ts file to these new versions.

[shiftkey]

@outofambit @iAmWillShepherd I'd love a sanity check that these changes make sense for y'all

[outofambit]

looks reasonable to me!, just one question