gitweb: Fix bug in insert_file() subroutine

gitweb: Fix bug in insert_file() subroutine

In insert_file() subroutine (which is used to insert HTML fragments as
custom header, footer, hometext (for projects list view), and per
project README.html (for summary view)) we used:

     map(to_utf8, <$fd>);

This doesn't work, and other form has to be used:

     map { to_utf8($_) } <$fd>;

Now with test for t9600 added, for $GIT_DIR/README.html.

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