Consistently use monospace font for commit SHAs and branch and tag names

As discussed in https://gitlab.slack.com/archives/C03MSG8B7/p1494430572778343.

Everything mostly looks the same, there's just no more non-monospace commit SHAs :)

Before

After

Look at Commit 5937ac0a in the header, master to the right of 1 parent, View file @ 5937ac0a on the right of .gitmodules, and Commit mention: 5937ac0a in the comment at the bottom.

/cc @tauriedavis