Broken gitHelpers.getNumberOfCommitsOnBranch on CircleCI

[patkub]

gitHelpers.getNumberOfCommitsOnBranch does not work on CircleCI.

The refactoring of gitHelpers.getNumberOfCommitsOnBranch in #35 changed its output on CircleCI.
Test: https://github.com/patkub/test-gk-lock-git-helpers/blob/master/index.js

On a greenkeeper branch, both methods fail and return 0.
CircleCI build: https://circleci.com/gh/patkub/test-gk-lock-git-helpers/35

Number of commits on branch: 0
Number of commits on branch (old method): 0

But, on the master branch, the old method properly detects the number of commits, while the new method returns 0.
CircleCI build: https://circleci.com/gh/patkub/test-gk-lock-git-helpers/33

Number of commits on branch: 0
Number of commits on branch (old method): 15

The old method prior to #35 was

greenkeeper-lockfile/lib/git-helpers.js

Lines 6 to 14 in e2e41a7

	getNumberOfCommitsOnBranch: function getNumberOfCommitsOnBranch (branch) {
	const grepAgument = `refs/heads/${branch}`
	const notArgument = `$(git for-each-ref --format="%(refname)" refs/heads/ | grep -v ${grepAgument})`
	return _.toNumber(
	exec(
	`git log ${branch} --oneline --not ${notArgument} | wc -l`
	).toString()
	)
	}

Current method is:

greenkeeper-lockfile/lib/git-helpers.js

Lines 8 to 16 in 6a1068a

	getNumberOfCommitsOnBranch: function getNumberOfCommitsOnBranch (branch) {
	const refArgument = `$(git for-each-ref '--format=%(refname)' refs/ | grep /${branch} | head -1)`
	const notArgument = `$(git for-each-ref '--format=%(refname)' refs/ | grep -v /${branch})`
	return _.toNumber(
	exec(
	`git log ${refArgument} --oneline --not ${notArgument} | wc -l`
	).toString()
	)
	},

[patkub]

@nevir Any thoughts on how to fix this for CircleCI and not potentially break other CIs?