Don't append co-authors to commit message more than once

Let's make sure that the output of `git mob-print` isn't already present
in the commit message before we append it.

This avoids an issue whereby amending a commit or interactive rebasing
could lead to the co-authors being added to the commit message
multiple times.

It would probably be even better to match each coauthor individually,
since coauthors could still be added multiple times if the mob changes,
but that's a bigger change and in the mean time this still seems like
an improvement.

PR: #52

Author: nickautomatic

hook-examples/prepare-commit-msg-nodejs

@@ -15,6 +15,10 @@ if(/COMMIT_EDITMSG/g.test(commitMessage)){
         // opens .git/COMMIT_EDITMSG
         contents = fs.readFileSync(commitMessage);
 
+        if(contents.indexOf(stdout.trim()) !== -1) {
+            process.exit(0);
+        }
+
         const commentPos = contents.indexOf('# ');
         const gitMessage = contents.slice(0, commentPos);
         const gitComments = contents.slice(commentPos)