Add a function call to retrieve the merge-base of two commit refs and return a commit object.
<rdar://problem/101292972>
Pull request: https://github.com/WebKit/WebKit/pull/5494
Committed 255689@main (402b4118973b): <https://commits.webkit.org/255689@main> Reviewed commits have been landed. Closing PR #5494 and removing active labels.
Committed 252432.600@safari-7614-branch (438d54807641): <https://commits.webkit.org/252432.600@safari-7614-branch> Reviewed commits have been landed. Closing PR #149 and removing active labels.