dashboard/builder: fix gccgo `git log` execution
dashboard/builder: fix gccgo `git log` execution

LGTM=adg
R=bradfitz
CC=adg, golang-codereviews
https://golang.org/cl/149630043

golang/build,golang/build,proppy/build,golang/build,golang/build,proppy/build,evandbrown/build,evandbrown/build,proppy/build,evandbrown/build,proppy/build