Certain complicated commit histories are highlighted incorrectly

[chiphogg]

This is for the "updates" view. Here is an example:

  *   Merge pull request #1148 from chrisbra/1131   Christian Brabandt, 6 months ago
  |\
  | * Make sure the statusline option is correct   Christian Brabandt, 6 months ago
  * |   Merge pull request #1152 from chrisbra/1146   Christian Brabandt, 6 months ago
  |\ \
  | * | Check that default value exists, before accessing it   Christian Brabandt, 6 months ago
  * | |   Merge pull request #1147 from hanxueluo/master   Christian Brabandt, 6 months ago
  |\ \ \
  | |_|/
  |/| |
  | * | shellescape file path when call "system()"   hanhuanle, 6 months ago
  | |/
  * |   Merge pull request #1145 from chrisbra/1144   Christian Brabandt, 6 months ago

The line with the _ is not recognized as depicting a commit tree, so it does not get highlighted.

I believe the fix should be simple; if so, I'll put out a pull request soon.

[ryanoasis]

related though not exactly is #791