Fix the contributors script to show only contributors on the current branch

Signed-off-by: Joffrey F <[email protected]>

script/release/contributors

@@ -15,10 +15,10 @@ EOM
 
 [[ -n "$1" ]] || usage
 PREV_RELEASE=$1
-VERSION=HEAD
+BRANCH="$(git rev-parse --abbrev-ref HEAD)"
 URL="https://api.github.com/repos/docker/compose/compare"
 
-contribs=$(curl -sf "$URL/$PREV_RELEASE...$VERSION" | \
+contribs=$(curl -sf "$URL/$PREV_RELEASE...$BRANCH" | \
     jq -r '.commits[].author.login' | \
     sort | \
     uniq -c | \