update a few Porcelain-ish for ref lock safety.

This updates the use of git-update-ref in git-branch, git-tag
and git-commit to make them safer in a few corner cases as
demonstration.

 - git-tag makes sure that the named tag does not exist, allows
   you to edit tag message and then creates the tag.  If a tag
   with the same name was created by somebody else in the
   meantime, it used to happily overwrote it.  Now it notices
   the situation.

 - git-branch -d and git-commit (for the initial commit) had the
   same issue but with smaller race window, which is plugged
   with this.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-branch.sh

@@ -42,8 +42,7 @@ If you are sure you want to delete it, run 'git branch -D $branch_name'."
 	    esac
 	    ;;
 	esac
-	rm -f "$GIT_DIR/logs/refs/heads/$branch_name"
-	rm -f "$GIT_DIR/refs/heads/$branch_name"
+	git update-ref -d "refs/heads/$branch_name" "$branch"
 	echo "Deleted branch $branch_name."
     done
     exit 0
@@ -112,6 +111,7 @@ rev=$(git-rev-parse --verify "$head") || exit
 git-check-ref-format "heads/$branchname" ||
 	die "we do not like '$branchname' as a branch name."
 
+prev=''
 if git-show-ref --verify --quiet -- "refs/heads/$branchname"
 then
 	if test '' = "$force"
@@ -121,10 +121,11 @@ then
 	then
 		die "cannot force-update the current branch."
 	fi
+	prev=`git rev-parse --verify "refs/heads/$branchname"`
 fi
 if test "$create_log" = 'yes'
 then
 	mkdir -p $(dirname "$GIT_DIR/logs/refs/heads/$branchname")
 	touch "$GIT_DIR/logs/refs/heads/$branchname"
 fi
-git update-ref -m "branch: Created from $head" "refs/heads/$branchname" $rev
+git update-ref -m "branch: Created from $head" "refs/heads/$branchname" "$rev" "$prev"

git-commit.sh

@@ -547,15 +547,15 @@ then
 		PARENTS=$(git-cat-file commit HEAD |
 			sed -n -e '/^$/q' -e 's/^parent /-p /p')
 	fi
-	current=$(git-rev-parse --verify HEAD)
+	current="$(git-rev-parse --verify HEAD)"
 else
 	if [ -z "$(git-ls-files)" ]; then
 		echo >&2 Nothing to commit
 		exit 1
 	fi
 	PARENTS=""
-	current=
 	rloga='commit (initial)'
+	current=''
 fi
 
 if test -z "$no_edit"
@@ -631,7 +631,7 @@ then
 	fi &&
 	commit=$(cat "$GIT_DIR"/COMMIT_MSG | git-commit-tree $tree $PARENTS) &&
 	rlogm=$(sed -e 1q "$GIT_DIR"/COMMIT_MSG) &&
-	git-update-ref -m "$rloga: $rlogm" HEAD $commit $current &&
+	git-update-ref -m "$rloga: $rlogm" HEAD $commit "$current" &&
 	rm -f -- "$GIT_DIR/MERGE_HEAD" &&
 	if test -f "$NEXT_INDEX"
 	then

git-tag.sh

@@ -63,8 +63,11 @@ done
 
 name="$1"
 [ "$name" ] || usage
-if [ -e "$GIT_DIR/refs/tags/$name" -a -z "$force" ]; then
-    die "tag '$name' already exists"
+prev=0000000000000000000000000000000000000000
+if test -e "$GIT_DIR/refs/tags/$name"
+then
+    test -n "$force" || die "tag '$name' already exists"
+    prev=`git rev-parse "refs/tags/$name"`
 fi
 shift
 git-check-ref-format "tags/$name" ||
@@ -109,4 +112,4 @@ fi
 
 leading=`expr "refs/tags/$name" : '\(.*\)/'` &&
 mkdir -p "$GIT_DIR/$leading" &&
-echo $object > "$GIT_DIR/refs/tags/$name"
+GIT_DIR="$GIT_DIR" git update-ref "refs/tags/$name" "$object" "$prev"