Move inline diff headers into a vertical column.
fix a couple of diff issues: shouldn't be any need to html encode before finding diffs; move quantifier outside regex condition
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.
ensure diff formatters escape their output
Merge remote-tracking branch 'whoopdedo/diffmarkup' into FS#2720
replace remaining hardcoded 'colspan' values with ->colspan
Merge pull request #179 from splitbrain/FS#2720

FS#2720, FS#2719