git-step-rebase: count distance to branch head correctly.

Without --first-parent, 'git rev-list' counts commits on merged
branches, too. Not noticed on Teacup, because we have no merges
here, but in another repo this came up.

tools/git-step-rebase

@@ -41,7 +41,7 @@ COMMON_PARENT=$(git merge-base HEAD ${BASE_BRANCH})
 echo
 echo "Common parent is ${COMMON_PARENT}."
 
-PARENTS=$(git rev-list ${COMMON_PARENT}..${BASE_BRANCH} | wc -l)
+PARENTS=$(git rev-list --first-parent ${COMMON_PARENT}..${BASE_BRANCH} | wc -l)
 echo "${PARENTS} commits to go."
 let PARENTS=${PARENTS}-1