tool/git-refresh: skip git fetch and checkout

if the tag is already checked out, to optimize the execution time.

I'm going to prepare a task depending on this tool, and I want that to
finish fast and output nothing when it's already up-to-date.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63883 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/git-refresh

@@ -31,6 +31,9 @@ dir="$2"
 shift 2
 [ x"$branch" = x ] && unset branch || :
 if [ -d "$dir" ]; then
+    if [ x"$(git -C "$dir" describe --tags)" = x"$branch" ]; then
+        exit 0 # already up-to-date
+    fi
     echo updating `expr "/$dir/" : '.*/\([^/][^/]*\)/'` ...
     [ $quiet ] || set -x
     $CHDIR "$dir"