git-node: refuse to run without configurations/on wrong revs

[joyeecheung]

It now refuses to run if

1. User has not configured upstream or branch - no more defaults.
2. User is not on the branch configured
3. User is on detached HEAD

Refs: nodejs/node#18914
Fixes: #198

[codecov]

Codecov Report

Merging #200 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #200   +/-   ##
=======================================
  Coverage   93.73%   93.73%           
=======================================
  Files          17       17           
  Lines         654      654           
=======================================
  Hits          613      613           
  Misses         41       41

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[bmeck]

would have been less confusing to me since I use default OSX git which I tend to land off git checkout upstream/master which this would show as detached head instead of confusion.

[gibfahn]

Maybe add a Fixes: https://github.com/nodejs/node-core-utils/issues/198