gitweb: Give side-by-side diff extra CSS styling

Use separate background colors for pure removal, pure addition and
change for side-by-side diff.  This makes reading such diff easier,
allowing to easily distinguish empty lines in diff from vertical
whitespace used to align chunk blocks.

Note that if lines in diff were numbered, the absence of line numbers
[for one side] would help in distinguishing empty lines from vertical
align.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>