Improve display behavior of commit messages in Diffusion

Summary:
See T372. Always render commit messages on one display line, so the table
doesn't jump around as they AJAX in on browse views.

The goal here is to have the cell choose a size naturally and for its content to
render with "overflow: hidden" if the natural size isn't large enough to contain
the content. "white-space: pre" or "white-space: nowrap" would prevent wrapping
but potentially make the table exceed the display width when a better behavior
is to hide some of the commit message.

Also use utf8-aware shortening, now that we have a function for it.

Casting a wide net in case anyone has a better way to do the CSS here. It's kind
of nasty that we have to use so many DOM nodes.

Test Plan:

• Resized window while viewing browse and history views in Safari, Chrome and

Firefox. Table exhibited described behavior.

• Verified summaries render sensibly and are properly truncated to 100

characters.

Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen, tomo, mroch, cpojer
CC: aran, epriestley
Differential Revision: 750