Move inline diff headers into a vertical column.
Monospace font for header prefix.
Fix width of indicator column.
Remove the prefix from inline diffs.

The line prefix was not actually indicating what it should indicate.
In particular, anything in a "changed" block would show a "!" even
if the entire line was added or deleted. Better to print nothing than
something that's wrong.
Merge remote-tracking branch 'whoopdedo/diffmarkup' into FS#2720