prepare-commit-msg: add 50 column marker

[emaste]

Summary

A common convention suggests a 50 column commit subject line (the first
line of the commit message). Add a 50 column marker to the commit
message template

[lwhsu]

How about adding a second comment line says that the 50 is for subject and 72 is for body?

[emaste]

How about adding a second comment line says that the 50 is for subject and 72 is for body?

I started writing some extra text but then it seems we should also comment on the blank 2nd line and it got to be a tutorial rather than a quick reference. Perhaps we could add a link to the section in the committer's guide. For now I'll commit this bit.