Summary of changes

• Fixed bug where signing the init commit failed due to being unable to update the HEAD ref.
• Changed NodeGit.Signature.default to async, because it actually ends up reading the config.
• Fixed bug where templates were not reporting errors for synchronous methods. It's a bit of a wide net, but in general, it is now possible certain sync methods in NodeGit will begin failin that did not fail before. This is the correct behavior.
• Switched NodeGit.Oid.fromString's internal implementation from git_oid_fromstr to git_oid_fromstrp
• Fixed builds for Electron 4
• Added NodeGit.Reference.updateTerminal

Merged PRs into NodeGit

• Fix non-existent / dangling refs cause Repository.prototype.createCommitWithSignature to fail #1624
• Handle new gyp information for electron builds #1623