Use helper methods to open activities.

Ensures getting the extras right.
Remove unused imports.
Fix 'View file at...'.

The commits API expects commit SHAs, not tree SHAs.
Fix overly long line.
Fix event line wrapping behaviour.

Setting the monospace font must be done in a separate span, as spans
changing the text metrics must subclass MetricAffectingSpan, which
ClickableSpan doesn't do.
Properly fix pull request comment SHA vs. ref mixup.

The original problem commit 31aea026ca48858dff345f42d6efc64011760f75
intended to fix was the usage of a file SHA as commit SHA. As we're
generally not interested in file SHAs, drop passing them around and only
use commit SHA, which we can later pass on as a tree ref to
FileViewerActivity if needed.