Recognize underscores as commit tree characters

[chiphogg]

Currently, any line in a commit tree representation with an underscore
will not be highlighted correctly. The solution is to add the
underscore to the list of commit tree characters.

Fixes #766.

[ryanoasis]

Thanks will check it out 👍

[davidlowryduda]

This request looks good to me. I suggest that it be merged.

[chiphogg]

Hi! Friendly ping @ryanoasis 😄

I just re-verified that it's easy to reproduce #766 by pasting the sample text into a new vim buffer, and running :setfiletype vundlelog. In the line with the underscore, everything after that character is highlighted incorrectly.

It's probably just as easy to verify the fix, by following the same procedure after patching in the branch, although I don't presently plan to re-test this unless requested.

For confidence, note that I was the original author of the files I'm modifying, in #633. So, it's very likely that the 2016 version of me knew what he was doing. 🙂

For motivation, note that merging this pull request will take care of two outstanding items on Milestone 0.10.3: one for the PR, one for the issue it resolves. This will take you from 3/8 to 5/8 progress---you'll be over the halfway mark!

Let me know if there's anything you need me to do.