fix: output error messages if git commands failed

[chinesedfan]

What:

Fixes #48.

Why:

The current usage of cb is not very suitable. It expects (err, ret), but buf/code is sent as err, which results in error.message to be undefined.

- git.stderr.on('data', cb)
- git.on('close', cb)
+ git.stderr.on('data', (buf) => {});
+ git.on('close', (code) => {})

How:

Collect all error messages and return.

Checklist:

• Documentation
• Tests
• Ready to be merged
• Added myself to contributors table

[codecov-io]

Codecov Report

Merging #102 into master will decrease coverage by 0.67%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage    58.5%   57.83%   -0.68%     
==========================================
  Files          19       19              
  Lines         429      434       +5     
  Branches       71       73       +2     
==========================================
  Hits          251      251              
- Misses        149      152       +3     
- Partials       29       31       +2

Impacted Files	Coverage Δ
src/util/git.js	23.52% <0%> (-4.06%)	⬇️

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[kentcdodds]

Seems like a good idea. Thanks!

[kentcdodds]

Would you like to add yourself to the contributors table?

[chinesedfan]

@kentcdodds I am already there, but forget to add code type this time. Never mind, I will send more PRs with myself as a coder.