gitweb: fix esc_param

The custom CGI escaping done in esc_param failed to escape UTF-8
properly. Fix by using CGI::escape on each sequence of matched
characters instead of sprintf()ing a custom escaping for each byte.

Additionally, the space -> + escape was being escaped due to greedy
matching on the first substitution. Fix by adding space to the
list of characters not handled on the first substitution.

Finally, remove an unnecessary escaping of the + sign.

Signed-off-by: Giuseppe Bilotta <redacted>
Signed-off-by: Junio C Hamano <redacted>