Fix event emitter leak by setting listener on process exit only once

[froucher]

This PR must resolve errors with eventEmitter memory leaks.

(node) warning: possible EventEmitter memory leak detected. 11 exit listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at process.addListener (events.js:179:15)
    at process.on.process.addListener (node.js:668:26)
    at spinner (/usr/local/lib/node_modules/gitwalk/node_modules/char-spinner/spin.js:35:13)
    at Object.getUpToDateRefs (/usr/local/lib/node_modules/gitwalk/out/lib/git.js:107:18)

The code was copied from two commits in another fork from atlassian (atlassian-forks@5c31a18 and atlassian-forks@330819a)

[stevenvachon]

Please fix broken tests.

[stevenvachon]

@froucher ^