Only show the first line of each commit in changelog dialog

[jryans]

Multi-line commits aren't actually formatted correctly, and most likely the
first line is enough of a summary anyway, so this change trims to the first
line. The commits are linked, so you can click through if you want more detail.

Fixes element-hq/element-web#8285

[bwindels]

lgtm, just thinking if the message could be null or undefined, but don't think so?

[jryans]

if the message could be null or undefined

git itself seems to abort if the message is empty... I am hoping all our commits will have messages, even if there is some way to force an empty one, so I think it's okay.