[ci] use better license-header impl that is not as cursed

Signed-off-by: crueter <crueter@eden-emu.dev>

.ci/license-header.sh

@@ -1,15 +1,11 @@
-#!/bin/bash -e
+#!/bin/sh -e
 
 HEADER="$(cat "$PWD/.ci/license/header.txt")"
 
 echo "Getting branch changes"
 
-BRANCH=`git rev-parse --abbrev-ref HEAD`
-COMMITS=`git log ${BRANCH} --not master --pretty=format:"%h"`
-RANGE="${COMMITS[${#COMMITS[@]}-1]}^..${COMMITS[0]}"
-FILES=`git diff-tree --no-commit-id --name-only ${RANGE} -r`
-
-#FILES=$(git diff --name-only master)
+BASE=`git merge-base master HEAD`
+FILES=`git diff --name-only $BASE`
 
 echo "Done"