Support Circle CI environment variables in Hatchet.git_branch

[edmorley]

Currently Hatchet.git_branch only recognises Travis CI environment variables, and for Circle CI has to fall back to querying Git:

hatchet/lib/hatchet.rb

Lines 28 to 38 in f97e6f5

	def self.git_branch
	# TRAVIS_BRANCH works fine unless the build is a pull-request. In that case, it will contain the target branch
	# not the actual pull-request branch! TRAVIS_PULL_REQUEST_BRANCH contains the correct branch but will be empty
	# for push builds. See: https://docs.travis-ci.com/user/environment-variables/
	return ENV['TRAVIS_PULL_REQUEST_BRANCH'] if ENV['TRAVIS_PULL_REQUEST_BRANCH'] && !ENV['TRAVIS_PULL_REQUEST_BRANCH'].empty?
	return ENV['TRAVIS_BRANCH'] if ENV['TRAVIS_BRANCH']
	out = `git rev-parse --abbrev-ref HEAD`.strip
	raise "Attempting to find current branch name. Error: Cannot describe git: #{out}" unless $?.success?
	out
	end

Circle sets the CIRCLE_BRANCH environment variable that could be used instead:
https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables

[schneems]

I think the original reason for the travis code was that it stripped the git repo or branch info maybe? Not shelling out will be a tiny perf benefit. I'm not sure if there's other benefits.

[edmorley]

I'm not sure if there's other benefits.

It avoids #180, though it's likely worth fixing that anyway for local use-cases.