gitweb: detect correct type for blobs < 512 bytes

gitweb: detect correct type for blobs < 512 bytes

The Perl -T/-B file operators return a wrong result when used on a pipe
that delivers less than 512 bytes before reaching end of file unless
the perlio layer is present.

Modern Perls generally are configured to use the perlio layer by
default.  However, some older Perls are configured to use the stdio
layer by default instead.

When dealing with blob data, check to see if the perlio layer is
present and add it if not so that we get the correct -T result for
small text blobs and do not send them back as an
application/octet-stream blob_plain file download.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>