gitweb: Highlight matched part of project name when searching projects

Use esc_html_match_hl() introduced in previous commit to escape HTML
and mark match, using span element with 'match' class.  Currently only
the 'path' part (i.e. the project name) is highlighted; match might be
on the project description.  Highlighting match in description is left
for next commit.

The code makes use of the fact that defined $search_regexp means that
there was search going on.

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