gitweb: Always use three argument form of open

In most cases (except insert_file() subroutine) we used old two argument
form of 'open' to open files for reading.  This can cause subtle bugs when
$projectroot or $projects_list file starts with mode characters ('>', '<',
'+<', '|', etc.) or with leading whitespace; and also when $projects_list
file or $mimetypes_file or ctags files end with trailing whitespace or '|'.

Additionally it is also more clear to explicitly state that we open those
files for reading.

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