gitweb: Highlight interesting parts of diff

Reading diff output is sometimes very hard, even if it's colored,
especially if lines differ only in few characters.  This is often true
when a commit fixes a typo or renames some variables or functions.

This commit teaches gitweb to highlight characters that are different
between old and new line with a light green/red background.  This should
work in the similar manner as in Trac or GitHub.

The algorithm that compares lines is based on contrib/diff-highlight.
Basically, it works by determining common prefix/suffix of corresponding
lines and highlightning only the middle part of lines.  For more
information, see contrib/diff-highlight/README.

Combined diffs are not supported but a following commit will change it.

Since we need to pass esc_html()'ed or esc_html_hl_regions()'ed lines to
format_diff_lines(), so it was taught to accept preformatted lines
passed as a reference.

Signed-off-by: Michał Kiedrowicz <redacted>
Acked-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>