Don't allow really long git-commit messages

[staxmanade]

A really large pull request in DefinitelyTyped/DefinitelyTyped#8605 caused every package to be re-published to NuGet.

At the end of the script is a git commit -m {msg} command. The {msg} was so big that it was throwing an error saying the command line command was too long to execute. Causing the system to not succeed and end up re-publishing all nuget packages every 2 hours (see image)

After running it once manually and manually committing the change in 6d2a959 it re-stabilized.

So we need to guard against a commit message that is too long.