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

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

Solution

Was there separate development on these branches? (the post you linked to, doesn't appear to have development on those branches) If there was no development, you could:

Checkout the branch git checkout branchName.
Tag it with git tag tagName.
Switch back to master git checkout master.
Finally, delete original branch with git branch branchName -d.

This can also be done if there was development on the branch, but you will need to use -D instead of -d. I'm not a git pro though, so not sure if that is an "acceptable" way to leave a branch.