refactor: allow non-empty stderr in runSync

[codebytere]

Refs #402 (comment).

Allow a non-empty stderr in runSync and instead error on a non-zero exit status; git sometimes outputs its messages to stderr so this has potential to cause unexpected failures.

cc @targos @joyeecheung

[codecov]

Codecov Report

Merging #410 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #410   +/-   ##
=======================================
  Coverage   76.28%   76.28%           
=======================================
  Files          21       21           
  Lines        1480     1480           
=======================================
  Hits         1129     1129           
  Misses        351      351           

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