Is there a simple command to convert a branch to a tag?

Question

I am about to complete a tedious process of converting "dumb snapshots" to git. This process has been going very well (thanks to this rename process), but now I realized that some of the branches that I created, do not merit a branch but rather a tag.

Since everything is still local (never pushed to a repository), I found this question (and associated answer) somewhat more cumbersome than I prefer, so I was wondering whether I can take a shortcut via some simple "convert-from-branch-to-tag" command?

Is there such a simple command to convert a branch to a tag?

(I know I can just leave it as is, but I really like the way gitk highlights tags, helping me easily identify them).

UPDATE: Thanks to @Andy's answer below, I managed to come up with a shell script that does it all conveniently and painlessly. I am sharing this script for the benefit of all and as special thanks to this great community who made moving from CVS to git possible for me:

#!/bin/sh

BRANCHNAME=$1
TAGNAME=$2

echo "Request to convert the branch ${BRANCHNAME} to a tag with the same name accepted."
echo "Processing..."
echo " "

git show-ref --verify --quiet refs/heads/${BRANCHNAME}
# $? == 0 means local branch with <branch-name> exists. 

if [ $? == 0 ]; then
   git checkout ${BRANCHNAME}
   git tag ${BRANCHNAME}
   git checkout master
   git branch ${BRANCHNAME} -d
   echo " "
   echo "Updated list branches, sorted chronologically: "
   echo "---------------------------------------------- "
   git log --no-walk --date-order --oneline --decorate $(git rev-list --branches --no-walk) | cut -d "(" -f 2 | cut -d ")" -f 1
else
   echo "Sorry. The branch ${BRANCHNAME} does NOT seem to exist. Exiting."
fi

Answer 1

The answers given are basically correct.

As tags and branches are just names for objects, there is a simpler way without touching current work area:

git tag <name_for_tag> refs/heads/<branch_name> # or just git tag <name_for_tag> <branch_name>
git branch -d <branch_name>

Or even do it to remote server without touching local repository at all:

git push origin origin/<branch_name>:refs/tags/<tag_name>
git push origin :refs/heads/<branch_name>