Exec GPG wrapper script from temp directory

[smashwilson]

Copy and execute the GPG wrapper script from the git-operation temporary directory rather than directly from the package source, so that it will work correctly when bundled in the Atom .asar file. This is mostly extracted from #846 so I'll have some nice, satisfying merge conflicts once I get back to that PR.

Fixes #1109.

[damieng]

Why is this hard-coded outside of Windows instead of just using os.tmpdir? Seems it would ignore the environment https://github.com/nodejs/node-v0.x-archive/blob/master/lib/os.js#L47

[smashwilson]

Oh, yeah, there's a long story here. The short version is that we create a Unix domain socket as a subdirectory of this, and on macOS and Linux, domain socket paths must be less than 104 or 108 bytes long. On macOS, os.tmpdir() returns a path beneath /private/ that eats up enough of the available space that (sometimes!) the domain socket creation will fail. So, we use os.tmpdir() on Windows and hard-code it to the nice, short /tmp on other OSes.

Note that this PR didn't actually introduce this, it just lost and restored the call by accident while I was doing git checkout -p from my other branch. The windows ternary is there on master:

github/lib/git-prompt-server.js

Lines 29 to 33 in 7835bc4

	this.tmpFolderPath = await getTempDir({
	dir: windows ? os.tmpdir() : '/tmp',
	prefix: 'github-',
	symlinkOk: true,
	});

Also, this is absolutely a hack, and I'd love to find a better way to ensure that the socket can be created successfully.

/cc @BinaryMuse who explained this to me the first time I saw this code 😉, in case she has anything to add