do_release.sh: Fix branching logic

tools/do_release.sh

@@ -1,7 +1,6 @@
 #!/bin/sh
 set -e
 
-SOURCE_BRANCH=$(git rev-parse --abbrev-ref HEAD)
 RELEASE_TRIGGER=${RELEASE_TRIGGER:-"Manual command-line makefile invocation"}
 
 set -x GIT_AUTHOR_NAME=${GIT_AUTHOR_NAME:-'STM32Square Auto-Release Script'}
@@ -15,7 +14,7 @@ fi
 TIMESTAMP=${TIMESTAMP:-$(date +%y-%m-%d)}
 
 git checkout release
-git merge -X theirs $SOURCE_BRANCH
+git merge -X theirs --no-commit master
 
 make clean all
 git add svd mx_mcu_db prog_db chip_db